How to Merge Two Eloquent Collections?

The merge method returns the merged collection, it doesn't mutate the original collection, thus you need to do the following

$original = new Collection(['foo']);

$latest = new Collection(['bar']);

$merged = $original->merge($latest); // Contains foo and bar.

Applying the example to your code

$related = new Collection();

foreach ($question->tags as $tag)
{
    $related = $related->merge($tag->questions);
}

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The merge() method on the Collection does not modify the collection on which it was called. It returns a new collection with the new data merged in. You would need:

$related = $related->merge($tag->questions);

However, I think you're tackling the problem from the wrong angle.

Since you're looking for questions that meet a certain criteria, it would probably be easier to query in that manner. The has() and whereHas() methods are used to generate a query based on the existence of a related record.

If you were just looking for questions that have any tag, you would use the has() method. Since you're looking for questions with a specific tag, you would use the whereHas() to add the condition.

So, if you want all the questions that have at least one tag with either 'Travel', 'Trains', or 'Culture', your query would look like:

$questions = Question::whereHas('tags', function($q) {
    $q->whereIn('name', ['Travel', 'Trains', 'Culture']);
})->get();

If you wanted all questions that had all three of those tags, your query would look like:

$questions = Question::whereHas('tags', function($q) {
    $q->where('name', 'Travel');
})->whereHas('tags', function($q) {
    $q->where('name', 'Trains');
})->whereHas('tags', function($q) {
    $q->where('name', 'Culture');
})->get();

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$users = User::all();
$associates = Associate::all();

$userAndAssociate = $users->merge($associates);

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Merge two different eloquent collections into one and some objects happen to have the same id, one will overwrite the other. Use push() method instead or rethink your approach to the problem to avoid that. Refer to web

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Creating a new base collection for each eloquent collection the merge works for me.

$foo = collect(Foo::all());
$bar = collect(Bar::all());
$merged = $foo->merge($bar);

In this case don't have conflits by its primary keys.

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I have faced some issue by using merge. So I used concat. You can used it like below.

$users = User::all();
$associates = Associate::all();
$userAndAssociate = $users->concat($associates);

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All do not work for me on eloquent collections, laravel eloquent collections use the key from the items I think which causes merging issues, you need to get the first collection back as an array, put that into a fresh collection and then push the others into the new collection;

public function getFixturesAttribute()
{
    $fixtures = collect( $this->homeFixtures->all() );
    $this->awayFixtures->each( function( $fixture ) use ( $fixtures ) {
        $fixtures->push( $fixture );
    });
    return $fixtures;
}

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I'm sorry about that, but since PHP 7.4 you're available to do like this (better use merge).

$foo = Foo::all();
$bar = Bar::all();

/** $foo will contain $foo + $bar */
$foo->push(...$bar);

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I would like to add that, i found that the concat method does not seem to override based on ID, while the merge method does. concat seems to work for me, while merge caused issues.

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