[10.x] Cross-database support for all relations

[bert-w]

Related to #23042

Issue

Various relationship methods do not work with cross database relations including withCount and whereHas. SQL errors were thrown because the database name was not prefixed in all cases. Some work has been done previously to accomplish this, however it wasn't tested thoroughly and several cases were left unconsidered (like joins and specific relation types).

Requirements

• Any MorphToMany/BelongsToMany relations are required to have an intermediary Eloquent model defined like so, because the connection from the intermediary table cant be resolved otherwise:

public function taggables()
{
    return $this->morphToMany(...)->using(Taggable::class);
}

• The rest of the relations work out of the box and any joins/froms that happen on a different connection automatically have their database name prefixed.

Task status

• Test all relationship methods (only correct SQL execution is tested)

Support status

• MySQL 5.7
• MySQL 8.0
• MariaDB 10.0
• SQL Server 2019 (FIXED prefixing dbo was required. A custom config override 'schema' has been added in the database configuration, similar to Postgres)
• NOT SUPPORTED: PostgreSQL 14 (it requires additional pgsql extensions dblink and/or postgres_fdw)
• NOT SUPPORTED: SQLite

[taylorotwell]

I don't make it a point or goal to support relationships across databases. I just don't think it's a good idea.

[bert-w]

@taylorotwell then why support it "somewhat" using the existing Illuminate\Database\Query::prependDatabaseNameIfCrossDatabaseQuery()?

There are countless usecases and reasons to support this (think of tenancy databases or a separate administration database that links to the main one). But most of all it is yet another reason for people to pick Laravel for their application.

[crissi]

@bert-w let me know if you ever make a package for this

[bert-w]

@crissi I've monkeypatched this myself by overriding a few composer files (you can define certain files in your composer.json to take precedence over the ones in Composer, then just add your own overrides in something like app\Overrides\Builder.php).

I dont think its practical to release this as a package since its not pretty to extend onto the existing Laravel classes and have them loaded instead of the default.

[nicholasbrantley]

@taylorotwell then why support it "somewhat" using the existing Illuminate\Database\Query::prependDatabaseNameIfCrossDatabaseQuery()?

There are countless usecases and reasons to support this (think of tenancy databases or a separate administration database that links to the main one). But most of all it is yet another reason for people to pick Laravel for their application.

the separate admin database is the use case I need this functionality for, and I'm requesting this be reconsidered. After using other built in cross database functionality like being able to set relations across models, the lack of being able to query with that relation is disappointing.

[byErikas]

Sorry for necroing, but I'm requesting this to be re-reviewed, and reconsidered. The support for this is already half-way existent, and would greatly assist many multi-db laravel users

[harryqt]

Laravel 12 (#54274) supports cross-database queries natively. 🚀