Difference between method calls $model->relation()

问题:

There is some basic understanding/theory here that I am missing.I don't understand the difference between these function calls:

$distributors = $store->distributors();
$distributors = $store->distributors;
$distributors = $store->distributors()->get();
$distributors = $store->distributors->get();

What I am trying to accomplis here is to get a list of the distributors for a store (a many to many relationship), and they get each distributors list of beers into one giant list.

foreach ($distributors as $distributor) 
{
    $available_beers = array_merge($distributor->beers(), $available_beers);
}

I don't know if that is the best way to do this and I can't get it to work. Similar to the first list of methods, I don't know if I need ->$beers or ->$beers()

Update

Thanks to everyone who answered! This will be a good reference for me going forward. My biggest lesson was the difference between getting a collection back, vs getting the query builder/relationship object back. For future reference to those who find this question, here is what I set up in my controller:

$store = $this->store->find($id)->first();
$distributors = $store->distributors;
$beers = [];
foreach ($distributors as $distributor){
    $beers = array_merge($distributor->beers->lists('name', 'id'), $beers);
}

回答1:

Short answer

$model->relation() returns the relationship object

$model->relation returns the result of the relationship

Long answer

$model->relation() can be explained pretty simple. You're calling the actual function you defined your relation with. Yours for distributor probably looks somewhat like this:

public function distributors(){
    return $this->hasMany('Distributor');
}

So when calling $store->distributors() you just get the return value of $this->hasMany('Distributor') which is an instance of Illuminate\Database\Eloquent\Relations\HasMany

When do you use it?

You usually would call the relationship function if you want to further specify the query before you run it. For example add a where statement:

$distributors = $store->distributors()->where('priority', '>', 4)->get();

Of course you can also just do this: $store->distributors()->get() but that has the same result as $store->distributors.

---

Which brings me to the explanation of the dynamic relationship property.

Laravel does some things under the hood to allow you to directly access the results of a relationship as property. Like: $model->relation.

Here's what happens in Illuminate\Database\Eloquent\Model

1) The properties don't actually exist. So if you access $store->distributors the call will be proxied to __get()

2) This method then calls getAttribute with the property name getAttribute('distributors')

public function __get($key)
{
    return $this->getAttribute($key);
}

3) In getAttribute it checks if the relationship is already loaded (exists in relations). If not and if a relationship method exists it will load the relation (getRelationshipFromMethod)

public function getAttribute($key)
{
    // code omitted for brevity

    if (array_key_exists($key, $this->relations))
    {
        return $this->relations[$key];
    }

    $camelKey = camel_case($key);

    if (method_exists($this, $camelKey))
    {
        return $this->getRelationshipFromMethod($key, $camelKey);
    }
}

4) In the end Laravel calls getResults() on the relation which then results in a get() on the query builder instance. (And that gives the same result as $model->relation()->get().

回答2:

The direct answer to your question:

• $store->distributors() will return the actual relationship object (\Illuminate\Database\Eloquent\Relations\BelongsToMany).
• $store->distributors will be a collection containing the results of the relationship query (\Illuminate\Database\Eloquent\Collection).
• $store->distributors()->get() will be a collection containing the results of the relationship query (\Illuminate\Database\Eloquent\Collection).
• $store->distributors->get() should return an error since you're calling get() on a Collection object and the first parameter is not optional. If not an error, it should at least return null.

More information:

Given the following model:

class Store extends Eloquent {
    public function distributors() {
        return $this->belongsToMany('Distributor');
    }
}

Calling the relationship method ($store->distributors()) will return to you the relationship (\Illuminate\Database\Eloquent\Relations\BelongsToMany) object. This is basically a query object which you can continue to modify, but you still need to call some type of method to get the results (e.g. get(), first(), etc).

However, accessing the relationship attribute ($store->distributors) will return to you a collection (\Illuminate\Database\Eloquent\Collection) object containing the results from executing the relationship query.

By default, the relationship attribute is created and assigned a value the first time it is accessed (known as "lazy loading"). So, the first time you access $store->distributors, behind the scenes it is executing the relationship query, storing the results in the $store->distributors attribute, and then returning those results. However, it only does this once. The next time you access $store->distributors, the attribute already contains the data, so that is what you are accessing.

To illustrate this:

// the following two statements will run the query twice
$r1 = $store->distributors()->get();
$r2 = $store->distributors()->get();

// the following two statements will run the query once.
// the first statement runs the query, populates $store->distributors, and assigns the variable
// the second statement just accesses the data now stored in $store->distributors
$r3 = $store->distributors;
$r4 = $store->distributors;

// at the end, $r1 == $r2 == $r3 == $r4

Relationships can also be "eager" loaded, using the with() method on the query. This is done to alleviate all of the extra queries that may be needed for lazy loading (known as the n+1 problem). You can read more about that here.

回答3:

When you work with relationships with Eloquent the property is a collection (Illuminate\Database\Eloquent\Collection) of your relation white the method is a start of a new query.

Say your model looks like this:

class User extends Eloquent {

    public function roles()
    {
        return $this->belongsToMany('Role');
    }

}

If you try to access $user->roles, Eloquent will run the query and fetch all roles related to that user thanks to magic methods and returns an instance of Illuminate\Database\Eloquent\Collection. That class has a method called get, that's why $user->roles->get() works for you.

If you try to access the method, $user->roles(), you will instead get a query builder object so you can fine tune your query.

$user->roles()->whereIn('role_id', [1, 3, 4])->get();

That would only return roles where role_id is 1, 3 or 4.

So, the property returns a complete query and it results (Illuminate\Database\Eloquent\Collection) while the method lets you customize your query.

回答4:

Maybe this will be usefull.

Access to method:

$object->method();

Access to property:

$object->property;

回答5:

$distributors = $store->distributors(); Result of a method (function)

$distributors = $store->distributors; Value of a property (variable)

$distributors = $store->distributors()->get(); Take the first one, where it's the result of a method, if the method returns an object, this is a method in that object that was returned.

$distributors = $store->distributors->get(); If the property is an object, then it's calling a method in that property that's an object.

Re ->$beers vs ->$beers() that's a dynamic name of a property/method depending on what you're for. Just making a really rough guess at what you're doing, in your class you're going to have

$this->beers = array('bud','miller','sam');

and in your code using the $store object, you're actually going to go something like

$drink_type = 'beers'; $drink_list = $store->$drink_type;

And that will return $this->beers from $store, the same as writing $store->beers;

回答6:

Imagine that the store class looks like this:

<?php

class Store {

    public $distributors;

    function __construct($distributors = array()) {
        $this->distributors = $distributors;
    }

    public function distributors() {
        return $this->distributors;
    }
}

So the difference is:

$store = new Store(array('some guy', 'some other guy'));
$guys = $store->distributors; # accesing the $distributors property
$more = $store->distributors(); # calling the distributors() method.

回答7:

The main difference is:

• $distributors = $store->distributors() return instance of the relationship object like Illuminate\Database\Eloquent\Relations\BelongsToMany. You can use other conditions such as where after call this.

• $store->distributors return instance of the collection Illuminate/Database/Eloquent/Collection. Laravel call the magic method __get under the hood. It will return a result of query relationship.