Laravel 4 : In nested relationship, how do I eager load and retrieve just the first/last data?

问题

A : User,
B : Conversations,
C : Messages

I want to get the conversation together with it's most recent message in an A->B->C relationship from A.

Working relationships in model

User belongsToMany Conversation
Conversation belongsToMany User
Conversation hasMany Message
Message belongsTo Conversation
User hasMany Message
Message belongsTo User

**note: User and Conversation are related to a many2many relationship connected 
by a pivot table

I've tried

$user->load(['conversations' => function($q)
            {
                $q->orderBy('created_at', 'desc');
                $q->with('Messages')->orderBy('created_at','desc');
            }]);

It nearly worked but it doesn't get the last message. Instead, it loads all of the messages belonging to that conversation.

Unwanted result from above code

array (size=7)
  'id' => int 90
  'name' => string 'haha,hehe' (length=9)
  'created_at' => string '2014-07-03 15:04:04' (length=19)
  'updated_at' => string '2014-07-03 15:04:04' (length=19)
  'microseconds' => string '1404370893302' (length=13)
  'pivot' => 
    array (size=2)
      'user_id' => int 1
      'conversations_id' => int 90
  'messages' => 
    array (size=27)
      0 => 
        ...
      1 => 
        ...
      ...
      26 => 
          array (size=7)
             'id' => int 816
             'user_id' => int 1
             'conv_id' => int 90
             'body' => string 'test1' (length=5)
             'created_at' => string '2014-07-03 01:13:35' (length=19)
             'updated_at' => string '2014-07-03 01:13:35' (length=19)
             'microseconds' => string '1404350015499' (length=13)

Expected result

array (size=7)
      'id' => int 90
      'name' => string 'haha,hehe' (length=9)
      'created_at' => string '2014-07-03 15:04:04' (length=19)
      'updated_at' => string '2014-07-03 15:04:04' (length=19)
      'microseconds' => string '1404370893302' (length=13)
      'pivot' => 
        array (size=2)
          'user_id' => int 1
          'conversations_id' => int 90
      'messages' => 
        array (size=1)
          0 => 
             array (size=7)
               'id' => int 816
               'user_id' => int 1
               'conv_id' => int 90
               'body' => string 'test1' (length=5)
               'created_at' => string '2014-07-03 01:13:35' (length=19)
               'updated_at' => string '2014-07-03 01:13:35' (length=19)
               'microseconds' => string '1404350015499' (length=13)

回答1:

When you put get (or first/find etc for that matter) in the closure it runs the query once more, but that query is ignored, and then Eloquent executes that query again:

$user->load(['conversations' => function($q)
{
   $q->orderBy('created_at', 'desc');
   $q->with('Messages')->orderBy('created_at','desc')
     ->get()  // this query is executed here
     ->first();  // this calls first method on the collection
     // but those 2 lines above take no effect with the eager load models
     // because Eloquent will execute the query again in the end
}]);

This is how you put constraints on the nested relation (but it won't do the job here):

User::with(['conversations.messages' => function ($q) {
  $q->orderBy(...)->where(...) ...;
  // but limit(1) can't be used here
}])->get();

Limit can't be used because it limits the whole set to 1, so there will be only 1 message returned in total. That said, only 1 conversation will have loaded single message in a collection.. Totally wrong

So the only way to achieve this with Eloquent is this 'helper' relation:

// Conversation model
public function latestMessage()
{
  return $this->hasOne('Message')->latest();
}

// then simply:

$user->load('conversations.latestMessage');

$user->conversations->first()->latestMessage; // single Message model

回答2:

Try using;

User::with('conversations', 'conversions.messages')

And with that you can add your eager loading contraints with a closure like;

User::with(array('conversations' => function(){}, 'conversions.messages' => function(){})

Be aware this is not tested.

来源：https://stackoverflow.com/questions/24550295/laravel-4-in-nested-relationship-how-do-i-eager-load-and-retrieve-just-the-fi

标签

php

laravel

laravel-4

eloquent