"PHP Error: Call to a member function getKey() on string" from diff() method on Laravel Collection

Question

Here are the dumped collections:

//$collOne
Illuminate\Database\Eloquent\Collection {#3386
  #items: array:6 [
    4807 => "{"color_id":7,"size_id":4,"pack_id":null}"
    4808 => "{"color_id":7,"size_id":2,"pack_id":null}"
    4809 => "{"color_id":7,"size_id":6,"pack_id":null}"
    4840 => "{"color_id":44,"size_id":4,"pack_id":null}"
    4841 => "{"color_id":44,"size_id":6,"pack_id":null}"
    4842 => "{"color_id":44,"size_id":2,"pack_id":null}"
  ]
}
//$collTwo
Illuminate\Database\Eloquent\Collection {#3403
  #items: array:5 [
    430 => "{"color_id":7,"size_id":4,"pack_id":null}"
    431 => "{"color_id":7,"size_id":2,"pack_id":null}"
    433 => "{"color_id":44,"size_id":4,"pack_id":null}"
    434 => "{"color_id":44,"size_id":6,"pack_id":null}"
    435 => "{"color_id":44,"size_id":2,"pack_id":null}"
  ]
}

Here is the error:

{
    "message": "Call to a member function getKey() on string",
    "exception": "Symfony\\Component\\Debug\\Exception\\FatalThrowableError",
    "file": "[obfuscated]/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Collection.php",
    "line": 281,
    "trace": [
        {
            "file": "[obfuscated].php",
            "line": 174,
            "function": "diff",
            "class": "Illuminate\\Database\\Eloquent\\Collection",
            "type": "->"
        },

Here is the line 174 referenced in the above error:

$diff = $collOne->diff($collTwo);

The docs seem pretty straight forward:

The diff method compares the collection against another collection or a plain PHP array based on its values. This method will return the values in the original collection that are not present in the given collection:

Am I missing something obvious here?

Blunk

Can you paste the entire stacktrace? Also, it would be interesting to understand how you got to the Eloquent collections - through a query or manually created? – Marked

The collections were formed with a mapWithKeys call on eloquent collections where I returned only specific properties and json encoded them in order to make comparisons on the two collections. – Blunk

oh dang, lol. It's the wheelmaker! small world eh? – Marked

haha, refresh my memory... did I get into a late night argument with you on something? I've been working on a project non-stop, 10+ hours a day for too many months to count now. – Blunk

Yea lol, techies I tell ya. Argue about everything from frameworks to approach to the "best" code :D – Marked

hahahahah, I remember now, the superfluous comment guy, may we should finish parsing that out first? ; ) – Blunk

If you could post your stack trace, I'll try and help here :) – Marked

Much appreciated, updated with what I can of the stacktrace, btw, turning the collections into arrays and using array_diff works perfectly fine. – Blunk

Illuminate\Database\Eloquent\Collection must be a collection of Eloquent models. I suspect this isn't. Can you post the output of dump($collOne->first()) and dump($collTwo->first()) – Marked

Answer 1

I suspect when you called mapWithKeys you did not return a Model instance. That's what's causing the error. Illuminate\Database\Eloquent\Collection must be a collection of Eloquent models but in this case it isn't.

A workaround could be calling diff in the base collection rather than the Eloquent collection like so:

$diff = $collOne->toBase()->diff($collTwo->toBase());

Or alternatively, when you call mapWithKeys, call it on the base collection like so:

$eloqCollection->toBase()->mapWithKeys(function($...) {...}) for both the collections. Then you can use the same code: $diff = $collOne->diff($collTwo); to compute the diff because now both $collOne and $collTwo would be instances of Illuminate\Support\Collection

Marked

nice catch @Marked – Blunk