Cross Database Joins

In Doctrine, joining data across databases is not technically "supported" by a designed feature, but you can make it work by tricking Doctrine a little bit.

In this article I'll show you how you can setup a database schema that specifies relationships across two databases and then issue a query which joins data from these two databases.

I used the Doctrine sandbox to prepare this test so if you want to try it, you can use it too.

Database Connections

First lets setup our two database connections we'll use to query from. Modify the config.php file included with the sandbox and replace the default single connection with the following code.

<?php

Doctrine_Manager::connection('mysql://root@localhost/doctrine_test1', 'doctrine_test1');
Doctrine_Manager::connection('mysql://root@localhost/doctrine_test2', 'doctrine_test2');

Schema

Now lets define our YAML schema file that we'll use to run our tests against. You can modify the config/doctrine/schema.yml file and include the following YAML.

---
User:
  tableName: doctrine_test1.user
  connection: doctrine_test1
  columns:
    username: string(255)
    password: string(255)

Profile:
  tableName: doctrine_test2.profile
  connection: doctrine_test2
  columns:
    user_id: integer
    first_name: string(255)
    last_name: string(255)
  relations:
    User:
      foreignType: one
      onDelete: CASCADE

Notice how we specify the full table name, including the name of the database. Currently, Doctrine does not generate the SQL that includes the database name. It only includes the table name, but we can trick Doctrine by simply specifying the database_name.table_name as the table name.

Test Data Fixtures

In the data/fixtures directory create a data.yml file and paste the following fixtures inside so we can have some data in each database to run our tests against.

---
User:
  jwage:
    username: jwage
    password: changeme
    Profile:
      first_name: string(255)
      last_name: string(255)

Build the Database

Now lets build our database and import the data fixtures from above. This can be easily done by running the following from the Doctrine command line interface.

$ php doctrine build-all-reload

Run the Test

Now we have our models created, we have our database created and we have our test fixtures loaded in to the database. Now it is time to run some sample code and see what we get!

First lets write our Doctrine_Query and look at the generated SQL. Paste the following code in to index.php and lets execute it!

<?php

$q = Doctrine::getTable('User')
  ->createQuery('u')
  ->leftJoin('u.Profile p');

echo $q->getSql();

The above code would output the following SQL query.

SELECT 
d.id AS d__id, 
d.username AS d__username, 
d.password AS d__password, 
d2.id AS d2__id, 
d2.user_id AS d2__user_id, 
d2.first_name AS d2__first_name, 
d2.last_name AS d2__last_name 
FROM doctrine_test1.user d 
LEFT JOIN doctrine_test2.profile d2 ON d.id = d2.user_id

Notice how in the above SQL that is generated it include the database name and the table name. So now the query is able to join across databases if your RDBMS supports it.

Now lets execute the above query and look at the results.

<?php

$q = Doctrine::getTable('User')
  ->createQuery('u')
  ->leftJoin('u.Profile p');

$users = $q->fetchArray();

print_r($users);

The above would output just exactly what you'd expect.

Array
(
    [0] => Array
        (
            [id] => 1
            [username] => jwage
            [password] => changeme
            [Profile] => Array
                (
                    [id] => 1
                    [user_id] => 1
                    [first_name] => string(255)
                    [last_name] => string(255)
                )

        )

)

The data from the User model came from one database, and the data from the Profile model came from the other database.

This will only work if your database supports foreign keys and joins across databases. I know MySQL does support this but I am unsure about others. This same method can be used to query for data across PostgreSQL schemas too.

That is it! Joining data from across different databases is no problem in Doctrine.

This is not a designed feature of Doctrine and you may experience edge cases that may not work as you'd expect. This is just useful if you need to join data across databases and if you experience edge cases you can work around them in your project.