Let's see how to search in the parent's fields using Laravel Scout and the database driver.

Let's say you have this relationship: users x cats. Each user has many cats (a "one-to-many" relationship):

class User extends Model
{
    public function cats()
    {
        return $this->hasMany(Cat::class);
    }
}

class Cat extends Model
{
    public function user()
    {
        return $this->belongsTo(User::class);
    }
}

Both models (users and cats) have a name field.

Let's say we want to get all cats with bob in their names, using Laravel's Scout.

The standard solution is to add this to the Cat.php model:

// Cat.php
use Searchable;

/**
  * Get the indexable data array for the model.
  *
  * @return array
  */
public function toSearchableArray()
{
    return [
        'name' => $this->name,
    ];
}

And we search with Cat::search('bob')->get().

This solution works well, but what if we want to search in the user fields?

The problem

Let's say we want to get cats owned by people with bob in their names.

If you add this to the "Cat" model:

// Cat.php
use Searchable;

/**
  * Get the indexable data array for the model.
  *
  * @return array
  */
public function toSearchableArray()
{
    // we only need to return keys for the database driver
    return [
        'name' => '', // cat's name
        'users.name' => '', // owner's name
    ];
}

It won't work. You will get this exception:

SQLSTATE[42S22]: Column not found: 1054 Unknown column 'users.name' in 'where clause'

SQL:
select `cats`.*
from `cats`
where (`cats`.`name` like %bob% or `users`.`name` like %bob%)

Clearly, the SQL is missing the users table. But how to add it?

Doing a Cat::join(...)->search('bob') will throw an exception, same for Cat::search(...)->join(...).

The question is: How to search in the parent attributes? And by "parent" I mean the "belongsTo" model.

Solution

The query method allows for modifing the search query. Use it to inject a join clause:

Cat::search('bob')->query(function ($builder) {
    $builder->join('users', 'cats.user_id', '=', 'users.id');
})->get();

This generates the proper query:

SELECT `cats`.*
FROM `cats`
INNER JOIN `users` on `cats`.`genre_id` = `users`.`id`
WHERE (`cats`.`name` LIKE '%bob%' or `users`.`name` LIKE '%bob%')
ORDER BY `id` desc

Automatically adding the JOIN clause on all searches

To avoid having to write ->join(...) on every search call, we can make Scout automatically add the JOIN:

// Cat.php
/**
 * Overrides the "search" method to inject a `join` to the relationships.
 */
use Searchable {
    Searchable::search as parentSearch;
}

/**
 * Perform a search against the model's indexed data.
 *
 * @param  string  $query
 * @param  \Closure  $callback
 * @return \Laravel\Scout\Builder
 */
public static function search($query = '', $callback = null)
{
    return static::parentSearch($query, $callback)->query(function ($builder) {
        $builder->join('users', 'cats.user_id', '=', 'users.id');
    });
}

Now, we just call Cat::search('bob')->get() to search for cats named "bob" or with owners named "bob".

Final code

class Cat extends Model
{
    /**
     * Overrides the "search" method to inject a `join` to the relationships.
     */
    use Searchable {
        Searchable::search as parentSearch;
    }

    /**
     * Get the indexable data array for the model.
    *
    * @return array
    */
    public function toSearchableArray()
    {
        // no need to return values as the database engine only uses the array keys
        return [
            'name' => '', // cat's name
            'users.name' => '', // owner's name
        ];
    }

    /**
     * Perform a search against the model's indexed data.
     *
     * @param  string  $query
     * @param  \Closure  $callback
     * @return \Laravel\Scout\Builder
     */
    public static function search($query = '', $callback = null)
    {
        return static::parentSearch($query, $callback)->query(function ($builder) {
            $builder->join('users', 'cats.user_id', '=', 'users.id');
        });
    }

    /**
     * Relationships
     */
    public function user()
    {
        return $this->belongsTo(User::class);
    }
}

And search with:

Cat::search('bob')->get();