[10.x] Fix an error when $query->from is of type Illuminate\Database\Query\Expression

[kztomita]

When passing a subquery referencing a different database to joinSub(), the prependDatabaseNameIfCrossDatabaseQuery() method would rewrite the "from" clause of the subquery. However, if $query->from is not a string but an Illuminate\Database\Query\Expression, the following error occurs:

TypeError: str_starts_with(): Argument #1 ($haystack) must be of type string, Illuminate\Database\Query\Expression given

To address this issue, I made modifications to prevent the replacement process when $query->from is an Illuminate\Database\Query\Expression instead of a string.

[taylorotwell]

Don't really have the bandwidth to chase this down at the moment. Don't pass an expression for now.

[MarFelix]

Hello, any other way to correct this error ?

[kztomita]

@MarFelix
To avoid this issue, I think the only way is to pass the subquery's FROM clause as a string rather than as an Expression.

Example:

    /*
    $subquery = $builder2
                  ->from(new Raw('database2.posts'))  // Expression
                  ->select('user_id, title');
    */
    $subquery = $builder2
                  ->from('posts')  // string
                  ->select('user_id, title');

    $builder = $this->getBuilder();
    $builder->from('users')->joinSub($subquery, 'sub', function ($join) {
        $join->on('users.id', '=', 'sub.user_id');
    });

Refer to the unit tests in https://github.com/laravel/framework/pull/49937/files.