MorphOneThrough Relationship

[JeffreyDavidson]

I know the MorphOneThrough relationship does not currently exist in the latest version of Laravel however I have a need for something like this.

Has anyone else had a need for something like this and can share how they were able to handle this?

[staudenmeir]

What do the tables involved look like?

[JeffreyDavidson]

@staudenmeir This is the tables and models involved.

Titles
id
name

ModelA
id
name

ModelB
id
name

TitleChampionships
title_id
champion_id
champion_type
won_at
lost_at

[staudenmeir]

You can actually build this relationship yourself:

class Title extends Model
{
    public function modelA()
    {
        return $this->hasOneThrough(ModelA::class, TitleChampionship::class, null, 'id', null, 'champion_id')
            ->where('champion_type', ModelA::class);
    }
}

[JeffreyDavidson]

@staudenmeir The only problem I have with that is that I would need it to be dynamic. Because there's no way of knowing what type it would be.

For example if a title has a current champion I would want it to grab the model that has a lost_at set to null but I wouldn't know if its going to return a ModelA or ModelB until it looks at the morphed entity.

$title->currentChampion. 

[JeffreyDavidson]

@staudenmeir Have you seen this situation before?

[staudenmeir]

Laravel doesn't support this kind of relationship because the SQL query would have to join a different table depending on the value of champion_type and that's not possible.

I created a package for complex cases like this that allows you to merge multiple relationships, but it requires a bit of setup:
https://github.com/staudenmeir/laravel-merged-relations

In your case, you would merge the two HasOneThrough relationships modelA and modelB. Also, the package currently doesn't offer a way to only return a single related result, only a collection. You would have to manually take the first collection item (e.g. with an accessor).

[staudenmeir]

Does this change if the consumer were to specify the possible values of champion_type?

@cwilby In a single relationship?

[cwilby]

@staudenmeir If I understand you correctly - yes, the polymorphic type would need to be defined per relationship instance.

A mechanic may work with many cars, where cars can morph to either one "Customer" or one "Dealership".

In that case, the mechanic model would define a "cars" relationship, where the $throughParents would be [Customer::class, Dealership::class].

There is potential for an extra select distinct owner_type from cars to determine all the possible types, I imagine most will want some way to constrain which types are included in the relationship.

[staudenmeir]

@cwilby You can absolutely build your own custom relation class with this approach. Is that what you mean?

Laravel doesn't support this kind of relationship

That might have been a bit ambiguous: I meant that you can't use a native Laravel relationship in this case.

[cwilby]

@cwilby You can absolutely build your own custom relation class with this approach. Is that what you mean?

Laravel doesn't support this kind of relationship

That might have been a bit ambiguous: I meant that you can't use a native Laravel relationship in this case.

Exactly 👍

For context - Jeff and I are collaborating on either adding a new relation to Laravel; adding to has-many-deep; or creating a new package.

[edalzell]

@JeffreyDavidson @cwilby was there any result of this? I need a morphOneThrough right now, what did you end up on?

[Tklaversma]

Any updates on this? I need a morphOneThrough as well...