A hands-on introduction to Wt::Dbo

For Wt 4.10.0

1. Introduction

Wt::Dbo is a C++ ORM (Object Relational Mapping) library.

The library is distributed as part of Wt for building database-driven web applications, but may be used independently of it as well.

The library provides a class-based view on database tables which keeps an object hierarchy of database objects automatically synchronized with a database by inserting, updating and deleting database records. C++ classes map to database tables, class fields to table columns, and pointers and collections of pointers to database relations. An object from a mapped class is called a database object (dbo). Query results may be defined in terms of database objects, primitives, or tuples of these.

A modern C++ approach is used to solve the mapping problem. Rather than resorting to XML-based descriptions of how C++ classes and fields should map onto tables and columns, or using obscure macros, the mapping is defined entirely in C++ code.

In this tutorial, we will work our way through a blogging example, similar to the one that is distributed with the library.

The complete source code for the examples used in this tutorial is available in the examples/feature/dbo/ folder of Wt.

2. Mapping a single class

We will start off with using Wt::Dbo for mapping a single class User to a corresponding table user.

In this tutorial and the examples, we alias the namespace Wt::Dbo to dbo, and in our explanation we will refer to types and methods available in that namespace directly.

To build the following example, you need to link against the wtdbo and wtdbosqlite3 libraries.

Mapping a single class (tutorial1.C)

#include <Wt/Dbo/Dbo.h>
#include <string>

namespace dbo = Wt::Dbo;

enum class Role {
    Visitor = 0,
    Admin = 1,
    Alien = 42
};

class User {
public:
    std::string name;
    std::string password;
    Role        role;
    int         karma;

    template<class Action>
    void persist(Action& a)
    {
        dbo::field(a, name,     "name");
        dbo::field(a, password, "password");
        dbo::field(a, role,     "role");
        dbo::field(a, karma,    "karma");
    }
};

This example shows how persistence support is defined for a C++ class. A template member method persist() is defined which serves as a persistence definition for the class. For each member in the class, a call to Wt::Dbo::field() is used to map the field to a table column name.

As you may notice, standard C++ types such as int, std::string and enum types are readily supported by the library (a full list of supported types can be found in the documentation of Wt::Dbo::sql_value_traits<T>). Support for other types can be added by specializing Wt::Dbo::sql_value_traits<T>. There is also support for built-in Wt types such as WDate, WDateTime, WTime and WString which can be enabled by including <Wt/Dbo/WtSqlTraits.h>.

The library defines a number of actions which will be applied to a database object using its persist() method, which applies it in turn to all its members. These actions will then read, update or insert database objects, create the schema, or propagate transaction outcomes.

For brevity, our example uses public members. There is nothing that prevents you from encapsulating your state in private members and providing accessor methods. You may even define the persistence method in terms of accessor methods by differentiating between read and write actions.

3. A first session

Now that we have a mapping definition for our User class, we can start a database session, create our schema (if necessary) and add a user to the database.

Let’s walk through the code for doing this.

(tutorial1.C continued)

void run()
{
    /*
     * Setup a session, would typically be done once at application startup.
     */
    auto sqlite3 = std::make_unique<dbo::backend::Sqlite3>("blog.db");
    dbo::Session session;
    session.setConnection(std::move(sqlite3));

    ...

The Session object is a long living object that provides access to our database objects. You will typically create a Session object for the entire lifetime of an application session, and one per user. None of the Wt::Dbo classes are thread safe (except for the connection pools), and session objects are not shared between sessions.

The lack of thread safety is not simply a consequence of laziness on our part. It coincides with the promises made by transactional integrity on the database: you will not want to see the changes made by one session in another session while its transaction has not been committed (Read-Committed transaction isolation level).

The session is given a connection which it may use to communicate with the database. A session will use a connection only during a transaction, and thus doesn’t really need a dedicated connection. When you’re planning for multiple concurrent sessions, it makes sense to use a connection pool instead, and a session may also be initialized with a reference to a connection pool.

Wt::Dbo uses an abstraction layer for database access, and currently supports PostgreSQL, Sqlite3, MySQL (or MariaDB), and Microsoft SQL Server as backends.

(tutorial1.C continued)

    ...

    session.mapClass<User>("user");

    /*
     * Try to create the schema (will fail if already exists).
     */
    session.createTables();

    ...

Next, we use mapClass() to register each database class with the session, indicating the database table onto which the class must be mapped.

Certainly during development, but also for initial deployment, it is convenient to let Wt::Dbo create or drop the database schema.

This generates the following SQL:

begin transaction;
create table "user" (
    "id" integer primary key autoincrement,
    "version" integer not null,
    "name" text not null,
    "password" text not null,
    "role" integer not null,
    "karma" integer not null
);
commit transaction;

As you can see, next to the four columns that map to C++ fields, by default, Wt::Dbo adds another two columns: id and version. The id is a surrogate primary key, and the version column is used for version-based optimistic locking. You can also suppress the version field, and provide natural keys of any type instead of the surrogate primary key, see Customizing the mapping.

Finally, we can add a user to the database. All database operations happen within a transaction.

(tutorial1.C continued)

    ...
    /*
     * A unit of work happens always within a transaction.
     */
    dbo::Transaction transaction(session);

    auto user = std::make_unique<User>();
    user->name = "Joe";
    user->password = "Secret";
    user->role = Role::Visitor;
    user->karma = 13;

    dbo::ptr<User> userPtr = session.add(std::move(user));
}

A call to Session::add() adds an object to the database. This call returns a ptr<User> to reference a database object of type User. This is a shared pointer which also keeps track of the persistence state of the referenced object. Within each session, a database object will be loaded at most once: the session keeps track of loaded database objects and returns an existing object whenever a query to the database requires this. When the last pointer to a database object goes out of scope, the transient (in-memory) copy of the database object is also deleted (unless it was modified, in which case the transient copy will only be deleted after changes have been successfully committed to the database).

The session also keeps track of objects that have been modified and which need to be flushed (using SQL statements) to the database. Flushing happens automatically when committing the transaction, or whenever needed to maintain consistency between the transient objects and the database copy (e.g. before doing a query).

The transaction commits automatically if the transaction object goes out of scope. If the transaction object goes out of scope due to an exception being thrown, the transaction will roll back instead.

This generates the following SQL:

begin transaction;
insert into "user" ("version", "name", "password", "role", "karma")
    values (?, ?, ?, ?, ?);
commit transaction;

All SQL statements are prepared once (per connection) and reused later, which has the benefit of avoiding SQL injection problems, and allows potentially better performance.

Because creating a dbo::ptr and then adding it to a session is such a common operation, there’s an addNew() shorthand, which acts as a make_unique followed by an add(). There’s also dbo::make_ptr, a make_unique-like function for dbo::ptr.

4. Querying objects

There are two ways of querying the database. Database objects of a single Dbo class can be queried using Session::find<Dbo>(condition):

(tutorial1.C continued)

dbo::ptr<User> joe = session.find<User>().where("name = ?").bind("Joe");

std::cerr << "Joe has karma: " << joe->karma << std::endl;

All queries use prepared statements with positional argument binding. The Session::find<T>() method returns a Query<ptr<T>> object. The Query object can be used to refine the search by defining SQL where, order by and group by definitions, and allows binding of parameters using Query::bind(). In this case the query should expect a single result and is cast directly to a database object pointer.

The Query class has a second parameter BindStrategy which has two possible values, corresponding to two different query implementations.

The default strategy is DynamicBinding and allows the query to be a long-lived object associated with the session which may be run multiple times. It also allows you to modify the query by changing only the order or the limit/offsets.

An alternative strategy is DirectBinding which passes bound parameters directly on to an underlying prepared statement. Such a query can be run only once, but has the benefit of having less (C++) overhead because the parameter values are directly passed on to the backend instead of stored within the query object.

The query formulated to the database is:

select "id", "version", "name", "password", "role", "karma"
    from "user"
    where (name = ?);

The more general way for querying uses Session::query<Result>(sql), which supports not only database objects as results. The above query is equivalent to:

(tutorial1.C continued)

dbo::ptr<User> joe2 = session.query<dbo::ptr<User>>("select u from user u")
    .where("name = ?").bind("Joe");

And this generates similar SQL:

select u."id" as col0, u."version" as col1, u."name" as col2,
       u."password" as col3, u."role" as col4, u."karma" as col5
    from user u
    where (name = ?);

The sql statement passed to the method may be arbitrary sql which returns results that are compatible with the Result type. The select part of the SQL query may be rewritten (as in the example above) to return the individual fields of a queried database object.

To illustrate that Session::query<Result>() may be used to return other types, consider the query below where an int result is returned.

(tutorial1.C continued)

int count = session.query<int>("select count(1) from user")
    .where("name = ?").bind("Joe");

The queries above were expecting unique results, but queries can also have multiple results. A Session::query<Result>() may therefore in general return a collection<Result> (for multiple results) and in the examples above they were coerced to a single unique Result for convenience. Similarly, Session::find<Dbo>() may return a collection<ptr<Dbo>> or a unique ptr<Dbo>. If a unique result is asked, but the query found multiple results, a NoUniqueResultException will be thrown.

collection<T> is an STL-compatible collection which has iterators that implement the InputIterator requirements. Thus, you can only iterate through the results of a collection once. After the results have been iterated the collection can no longer be used (but the Query object can be reused unless a DirectBinding bind strategy was used).

The following code shows how multiple results of a query may be iterated:

(tutorial1.C continued)

using Users = dbo::collection<dbo::ptr<User>>;

Users users = session.find<User>();

std::cerr << "We have " << users.size() << " users:" << std::endl;

for (const dbo::ptr<User> &user : users) {
    std::cerr << " user " << user->name
              << " with karma of " << user->karma << std::endl;
}

This code will perform two database queries: one for the call to collection::size() and one for iterating the results:

select count(1) from (select "id", "version", "name", "password", "role", "karma" from "user");
select "id", "version", "name", "password", "role", "karma" from "user";

5. Updating objects

Unlike most other smart pointers, ptr<Dbo> is read-only by default: dereferencing it yields a const Dbo&. To modify a database object, you need to call the ptr::modify() method, which returns a non-const object. This marks the object as dirty and the modifications will later be synchronized to the database.

(tutorial1.C continued)

dbo::ptr<User> joe = session.find<User>().where("name = ?").bind("Joe");

joe.modify()->karma++;
joe.modify()->password = "public";

Database synchronization doesn’t happen instantaneously. Instead, they’re delayed until:

They are explicitly flushed, using ptr<Dbo>::flush() or Session::flush()
A query is executed whose results may be affected by the changes
The transaction is committed

The previous code will generate the following SQL:

select "id", "version", "name", "password", "role", "karma"
    from "user"
    where (name = ?);
update "user"
    set "version" = ?, "name" = ?, "password" = ?, "role" = ?, "karma" = ?
    where "id" = ? and "version" = ?;

We already saw how using Session::add(ptr<Dbo>), we added a new object to the database. The opposite operation is ptr<Dbo>::remove(): it deletes the object in the database.

(tutorial1.C continued)

dbo::ptr<User> joe = session.find<User>().where("name = ?").bind("Joe");

joe.remove();

After removing an object, the transient object can still be used, and can even be re-added to the database.

Like modify(), also the add() and remove() operations defer synchronization with the database, and therefore the following code doesn’t actually have any effect on the database:

(tutorial1.C continued)

dbo::ptr<User> silly = session.addNew<User>();
silly.modify()->name = "Silly";
silly.remove();

6. Mapping relations

6.1. Many-to-One relations

Let’s add posts to our blogging example, and define a Many-to-One relation between posts and users. In the code below, we limit ourselves to the statements important for defining the relationship.

Many-to-One relation (tutorial2.C)

#include <Wt/Dbo/Dbo.h>
#include <string>

namespace dbo = Wt::Dbo;

class User;

class Post {
public:
    ...

    dbo::ptr<User> user;

    template<class Action>
    void persist(Action& a)
    {
        ...

        dbo::belongsTo(a, user, "user");
    }
};

class User {
public:
    ...

    dbo::collection< dbo::ptr<Post> > posts;

    template<class Action>
    void persist(Action& a)
    {
        ...

        dbo::hasMany(a, posts, dbo::ManyToOne, "user");
    }
};

On the Many side, we add a reference to a user, and in the persist() method we call belongsTo(). This allows us to reference the user to which this post belongs. The last argument will correspond to the name of the database column which defines the relationship.

On the One side, we add a collection of posts, and in the persist() method we call hasMany(). The join field must be the same name as the corresponding belongsTo() call.

If we add the Post class to our session using Session::mapClass(), and create the schema, the following SQL is generated:

create table "user" (
    ...

    -- table user is unaffected by the relationship
);

create table "post" (
    ...

    "user_id" bigint,
    constraint "fk_post_user" foreign key ("user_id") references "user" ("id")
        deferrable initially deferred
);

Note the user_id field which corresponds to the join name "user".

On the Many side, you may read or write the ptr to set a user to whom this post belongs.

The collection on the One side allows us to retrieve all associated elements, and also insert() and remove() elements, which has the same effect as setting the ptr on the Many side.

Example:

(tutorial2.C continued)

dbo::ptr<Post> post = session.addNew<Post>();
post.modify()->user = joe; // or joe.modify()->posts.insert(post);

// will print 'Joe has 1 post(s).'
std::cerr << "Joe has " << joe->posts.size() << " post(s)." << std::endl;

As you can see, as soon as joe is set as user for the new post, the post is reflected in the posts collection of joe, and vice-versa.

6.2. Many-to-Many relations

To illustrate Many-to-Many relations, we will add tags to our blogging example, and define a Many-to-Many relation between posts and tags. In the code below, we again limit ourselves to the statements important for defining the relationship.

Many-to-Many relation (tutorial2.C)

#include <Wt/Dbo/Dbo.h>
#include <string>

namespace dbo = Wt::Dbo;

class Tag;

class Post {
public:
    ...

    dbo::collection< dbo::ptr<Tag> > tags;

    template<class Action>
    void persist(Action& a)
    {
        ...

        dbo::hasMany(a, tags, dbo::ManyToMany, "post_tags");
    }
};

class Tag {
public:
    ...

    dbo::collection< dbo::ptr<Post> > posts;

    template<class Action>
    void persist(Action& a)
    {
        ...

        dbo::hasMany(a, posts, dbo::ManyToMany, "post_tags");
    }
};

As expected, the relationship is reflected in almost the same way in both classes: they both have a collection of database objects of the related class, and in the persist() method we call hasMany(). The join field in this case will correspond to the name of a join table used to persist the relation.

Adding the Post class to our session using Session::mapClass(), we now get the following SQL for creating the schema:

create table "post" (
    ...

    -- table post is unaffected by the relationship
);

create table "tag" (
    ...

    -- table tag is unaffected by the relationship
);

create table "post_tags" (
    "post_id" bigint not null,
    "tag_id" bigint not null,
    primary key ("post_id", "tag_id"),
    constraint "fk_post_tags_key1" foreign key ("post_id")
        references "post" ("id") on delete cascade deferrable initially deferred,
    constraint "fk_post_tags_key2" foreign key ("tag_id")
        references "tag" ("id") on delete cascade deferrable initially deferred
);

create index "post_tags_post" on "post_tags" ("post_id");
create index "post_tags_tag" on "post_tags" ("tag_id");

The collection on either side of the Many-to-Many relation allows us to retrieve all associated elements. To define a relation between a post and a tag, you need to add the post to the tag’s posts collection, or the tag to the post’s tags collection. You may not do both! The change will automatically be reflected in the reciprocal collection. Likewise, to undo the relation between a post and a tag, you should remove the tag from the post’s tags collection, or the post from the tag’s posts collection, but not both.

Example:

(tutorial2.C continued)

dbo::ptr<Post> post = ...
dbo::ptr<Tag> cooking = session.addNew<Tag>();
cooking.modify()->name = "Cooking";

post.modify()->tags.insert(cooking);

// will print '1 post(s) tagged with Cooking.'
std::cerr << cooking->posts.size() << " post(s) tagged with Cooking."
          << std::endl;

6.3. One-to-One relations

Let’s add a Settings class to our blogging example, and define a One-to-One relation between settings and users. In the code below, we limit ourselves to the statements important for defining the relationship.

One-to-One relation (tutorial2.C)

#include <Wt/Dbo/Dbo.h>
#include <string>

namespace dbo = Wt::Dbo;

class User;

class Settings {
public:
    ...

    dbo::ptr<User> user;

    template<class Action>
    void persist(Action& a)
    {
        ...

        dbo::belongsTo(a, user);
    }
};

class User {
public:
    ...

    dbo::weak_ptr<Settings> settings;

    template<class Action>
    void persist(Action& a)
    {
        ...

        dbo::hasOne(a, settings);
    }
};

Although a One-to-One relation sounds symmetrical, its implementation in a database and Wt::Dbo isn’t. In the database, the relation is defined by a foreign key from one table to the other (in our example, from settings to user). We’ll differentiate between both sides by stating that one side is owning, and the other side is owned.

On the owned side, we add a reference to a user, and in the persist() method we call belongsTo(). This allows us to reference the user to which these settings belong.

On the owning side, we add a weak reference to its settings, and in the persist() method we call hasOne().

If we add the Settings class to our session using Session::mapClass(), and create the schema, the following SQL is generated:

create table "user" (
    ...

    -- table user is unaffected by the relationship
);

create table "settings" (
    ...

    "user_id" bigint,
    constraint "fk_settings_user" foreign key ("user_id") references "user" ("id")
        deferrable initially deferred
)

At the owning side, we use a weak_ptr to avoid creating a cycle. The weak_ptr doesn’t actually store the reference (nor does the underlying database record), but is defined instead in terms of a database query. This has as a consequence that any operation on it will involve a query.

On either side, you may change the value, and this will update the reciprocal side of the relationship. Example:

(tutorial2.C continued)

dbo::ptr<User> joe = session.find<User>().where("name = ?").bind("Joe");

dbo::ptr<Settings> settings = session.addNew<Settings>();
settings.modify()->theme = "fancy-pink";
joe.modify()->settings = settings;

// will print 'Settings apply to Joe'
std::cerr << "Settings apply to " << settings->user->name << std::endl;

As you can see, as soon as one side of the relation is modified, this is reflected in the other side as well.

7. Customizing the mapping

By default, Wt::Dbo will add an auto-incrementing surrogate primary (id) key and a version field (version) to each mapped table.

While these defaults make sense for a new project, you can tailor the mapping so that you can map to virtually any existing database schema.

7.1. Changing or disabling the surrogate primary key "id" field

To change the field name used for the surrogate primary key for a mapped class, or to disable the surrogate primary key for a class and use a natural key instead, you need to specialize Wt::Dbo::dbo_traits<C>.

For example, the code below changes the primary key field for class Post from id to post_id:

Changing the "id" field name (tutorial3.C)

#include <Wt/Dbo/Dbo.h>

namespace dbo = Wt::Dbo;

class Post {
public:
  ...
};

namespace Wt {
    namespace Dbo {

        template<>
        struct dbo_traits<Post> : public dbo_default_traits {
            static const char *surrogateIdField() {
                return "post_id";
            }
        };

    }
}

7.2. Changing or disabling the "version" field

To change the field name used for the optimistic concurrency control version field (version), or to disable optimistic concurrency control for a class altogether, you need to specialize Wt::Dbo::dbo_traits<C>.

For example, the code below disables optimistic concurrency control for class Post:

Disabling the "version" field name (tutorial4.C)

#include <Wt/Dbo/Dbo.h>

namespace dbo = Wt::Dbo;

class Post {
public:
    ...
};

namespace Wt {
    namespace Dbo {

        template<>
        struct dbo_traits<Post> : public dbo_default_traits {
            static const char *versionField() {
                return nullptr;
            }
        };

    }
}

7.3. Specifying a natural primary key

Instead of using a auto-incrementing surrogate primary key, you may want to use a different primary key.

For example, the code below changes the primary key for the User table to a string (their username) which maps onto a varchar (20) field user_name:

Using a natural key (tutorial5.C)

#include <Wt/Dbo/Dbo.h>

namespace dbo = Wt::Dbo;

class User {
public:
    std::string userId;

    template<class Action>
    void persist(Action& a)
    {
        dbo::id(a, userId, "user_id", 20);
    }
};

namespace Wt {
    namespace Dbo {

        template<>
        struct dbo_traits<User> : public dbo_default_traits {
            using IdType = std::string;

            static IdType invalidId() {
                return std::string();
            }

            static const char *surrogateIdField() { return nullptr; }
        };

    }
}

The id() function has the same syntax as the field() function.

A natural primary key may also be a composite key, a foreign key or a combination.

7.4. Specifying a composite natural primary key

To use a composite type as a natural primary key, i.e. a type which consists of more than one field, you need to have a corresponding C++ type.

The type has a number of basic requirements, such as default constructor, comparison operators (== and <), and a streaming operator.

Using a composite natural primary key (tutorial6.C)

struct Coordinate {
    int x, y;

    Coordinate()
        : x(-1), y(-1) { }

    Coordinate(int an_x, int an_y)
        : x(an_x), y(an_y) { }

    bool operator== (const Coordinate& other) const {
        return x == other.x && y == other.y;
    }

    bool operator< (const Coordinate& other) const {
        if (x < other.x)
            return true;
        else if (x == other.x)
            return y < other.y;
        else
            return false;
    }
};

std::ostream& operator<< (std::ostream& o, const Coordinate& c)
{
    return o << "(" << c.x << ", " << c.y << ")";
}

Next, you must indicate how the type is persisted, by overloading Dbo’s field() function for it.

(tutorial6.C continued)

namespace Wt {
    namespace Dbo {

        template <class Action>
        void field(Action& action, Coordinate& coordinate,
                   const std::string& name, int size = -1)
        {
            field(action, coordinate.x, name + "_x");
            field(action, coordinate.y, name + "_y");
        }
    }
}

With this in place, we can use the Coordinate type as a natural primary key type:

(tutorial6.C continued)

class GeoTag;

namespace Wt {
    namespace Dbo {

         template<>
         struct dbo_traits<GeoTag> : public dbo_default_traits
         {
             using IdType = Coordinate;
             static IdType invalidId() { return Coordinate{}; }
             static const char *surrogateIdField() { return nullptr; }
         };
    }
}

class GeoTag {
public:
     Coordinate  position;
     std::string name;

     template <class Action>
     void persist(Action& a)
     {
          dbo::id(a, position, "position");
          dbo::field(a, name, "name");
     }
};

Note that the composite key may also include foreign keys, by storing ptr<> objects in the composite, which you map using a belongsTo() declaration. See tutorial8.C for a complete example.

7.5. Specifying foreign key constraints

The belongsTo() function is overloaded so that you can add foreign key constraints which are enforced by the database, such as:

NotNull: can’t be null
OnUpdateCascade: cascade an update of the (natural) primary key to the foreign keys that reference it
OnUpdateSetNull: an update of the (natural) primary key sets referencing foreign keys to null
OnDeleteCascade: cascade a delete of the object to also delete objects that reference it using a foreign key
OnDeleteSetNull: when the object is deleted, set the referencing foreign keys to null.

In the next chapter we will see how you can specify these foreign key constraints also for foreign keys that double as primary key.

7.6. Specifying a natural primary key that is also a foreign key

Let’s define a class UserInfo which provides additional data for a User. We will only allow exactly one UserInfo object per User, and therefore choose a reference to the User as the primary key for the UserInfo.

Using a foreign key as primary key (tutorial7.C)

#include <Wt/Dbo/Dbo.h>
#include <Wt/Dbo/backend/Sqlite3.h>

namespace dbo = Wt::Dbo;

class UserInfo;
class User;

namespace Wt {
    namespace Dbo {

        template<>
        struct dbo_traits<UserInfo> : public dbo_default_traits {
            using IdType = ptr<User>;

            static IdType invalidId() {
                return ptr<User>{};
            }

            static const char *surrogateIdField() { return nullptr; }
        };

    }
}

class User
{
public:
    std::string name;

    dbo::weak_ptr<UserInfo> info;

    template<class Action>
    void persist(Action& a)
    {
         dbo::field(a, name, "name");

         dbo::hasOne(a, info, "user");
    }
};

class UserInfo
{
public:
    dbo::ptr<User> user;
    std::string info;

    template<class Action>
    void persist(Action& a)
    {
        dbo::id(a, user, "user", dbo::OnDeleteCascade);
        dbo::field(a, info, "info");
    }
};

void run()
{
    /*
     * Setup a session, would typically be done once at application startup.
     */
    auto sqlite3 = std::make_unique<dbo::backend::Sqlite3>(":memory:");
    sqlite3->setProperty("show-queries", "true");
    dbo::Session session;
    session.setConnection(std::move(sqlite3));

    session.mapClass<User>("user");
    session.mapClass<UserInfo>("user_info");

    /*
     * Try to create the schema (will fail if already exists).
     */
    session.createTables();

    {
        dbo::Transaction transaction{session};

        auto user = std::make_unique<User>();
        user->name = "Joe";

        dbo::ptr<User> userPtr = session.add(std::move(user));

        auto userInfo = std::make_unique<UserInfo>();
        userInfo->user = userPtr;
        userInfo->info = "great guy";

        session.add(std::move(userInfo));
    }

    {
        dbo::Transaction transaction{session};

        dbo::ptr<UserInfo> info = session.find<UserInfo>();

        std::cerr << info->user->name << " is a " << info->info << std::endl;
    }
}

int main(int argc, char **argv)
{
    run();
}

As you can see, in this example, we would really need a One-to-One relationship, but this is currently not yet supported in Dbo, and thus we emulate it using a Many-to-One relationship (which has the same representation in SQL).

When run, this should output:

begin transaction;
create table "user" (
    "id" integer primary key autoincrement,
    "version" integer not null,
    "name" text not null
);

create table "user_info" (
    "version" integer not null,
    "user_id" bigint,
    "info" text not null,
    primary key ("user_id"),
    constraint "fk_user_info_user" foreign key ("user_id")
        references "user" ("id") on delete cascade
        deferrable initially deferred
);

commit transaction;
begin transaction;
insert into "user" ("version", "name") values (?, ?);
insert into "user_info" ("version", "user_id", "info") values (?, ?, ?);
commit transaction;
begin transaction;
select version, "user_id", "info" from "user_info";
select "version", "name" from "user" where "id" = ?;
commit transaction;

8. Transactions and concurrency

Reading data from the database or flushing changes to the database requires an active transaction. A Transaction is an RAII (Resource Acquisition is Initialization) class which at the same time provides isolation between concurrent sessions and atomicity for persisting changes to the database.

The library implements optimistic locking, which allows detection (rather than avoidance) of concurrent modifications. It is a recommended and widely used strategy for dealing with concurrency issues in a scalable manner as no write locks are needed on the database. To detect a concurrent modification, a version field is added to each table which is incremented on each modification. When performing a modification (such as updating or removing an object), it is checked that the version of the record in the database is the same as the version of the object that was originally read from the database.

Transaction isolation levels

The minimum level of isolation which is required for the library’s optimistic locking strategy is Read Committed: modifications in a transaction are only visible to other sessions as soon as they’re committed. This is usually the lowest level of isolation supported by a database.

The Transaction class is a lightweight proxy that references a logical transaction: multiple (usually nested) Transaction objects may be instantiated simultaneously, which each need to be committed for the logical transaction to be committed. In this way you can easily protect individual methods which require database access with such a transaction object, which will automatically participate in a wider transaction if that is available. A transaction will in fact defer opening a real transaction in the database until needed, and thus there is no penalty for instantiating a transaction to make sure a unit of work is atomic, even if you aren’t yet sure that there will be actual work done. Note that there is no need to explicitly commit a transaction: a transaction will automatically commited when it goes out of scope, unless the transaction goes out of scope (and thus its destructor is called) while an exception is being thrown.

Transactions may fail and dealing with failing transactions is an integral aspect of their usage. When the library detects a concurrent modification, a StaleObjectException is thrown. Other exceptions may be thrown, including exceptions in the backend driver when for example the database schema is not compatible with the mapping. There may also be problems detected by the business logic which may raise an exception and cause the transaction to be rolled back. When a transaction is rolled back, the modified database objects aren’t successfully synchronized with the database, but may be synchronized later in a new transaction.

Obviously, many exceptions will be fatal. One notable exception is the StaleObjectException however. Different strategies are possible to deal with this exception. Regardless of the approach, you will at least need to reread() the stale database object(s) before being able to commit changes made in a new transaction.

9. Installation

Wt::Dbo is included in Wt and can thus be installed as part of this library for which there may be standard packages available for your operating system.

The library doesn’t depend in any way on Wt, however, and can also be built, installed and used separately from it. Starting from a Wt source package (and on a UNIX-like environment), you would do the following to build and install only Wt::Dbo:

Installing Wt::Dbo from source (UNIX-like)

$ cd wt-xxx
$ mkdir build
$ cd build
$ cmake ../ # extra options may be needed, see instructions
$ cd src/Wt/Dbo
$ make
$ sudo make install

See also the Wt installation instructions.