How to Merge Two Eloquent Collections?

Question

I have a questions table and a tags table. I want to fetch all questions from tags of a given question. So, for example, I may have the tags "Travel," "Trains" and "Culture" attached to a given question. I want to be able to fetch all questions for those three tags. The tricky, so it seems, is that questions and tags relationship is a many-to-many defined in Eloquent as belongsToMany.

I thought about trying to merge the questions Collections as below:

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

It doesn't seem to work though. Doesn't seem to merge anything. Am I attempting this correctly? Also, is there perhaps a better way to fetch a row of rows in a many-to-many relationship in Eloquent?

Answer 1

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); } 

Answer 2

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();