问题描述
我有以下三个表:
A
-------------
| id | name |
-------------
B
--------------------
| id | A_id | name |
--------------------
C
--------------------
| id | B_id | name |
--------------------
所以，表C中的数据属于表B中的数据，而表A中的数据属于表A中的数据.现在，我想查询 C，同时还从 B 和 A 检索数据，下面的代码就可以了.
So, the data in table C belongs to the data in table B which belongs to the data in table A. Now, I want to query C, while also retrieving data from B and A and the following code does the trick just fine.
C::with('B.A')->get();
现在的问题是，我想用一些约束查询 C.这些约束之一是 A 的 id.我尝试了以下方法:
The problem now, is that I want to query C with some constraints. One of these constraints is the id of A. I've tried the following:
C::with(array('B.A' => function ($query)
{
$query->where('id', '=', $constraint);
}))->get();
但似乎 Eloquent 将检索 C 中的所有行，甚至不考虑约束，除非它执行查询以检索表 A 中的数据.我该如何解决这个问题?我是否需要在 C 中添加另一个字段，即 A_id，并将 $constraint 与该字段匹配?
But it seems that Eloquent will retrieve all the rows in C without even taking the constraint into account, except when it's executing the query to retrieve data in table A. How do I get around this problem? Do I need to add another field in C, that is A_id, and match $constraint against that field?
推荐答案
您将 with() 方法与 SQL 的 JOIN 方法混淆了，这种情况经常发生.
You are confusing the with() method with SQL's JOIN and that happens a lot.
当你使用 Foo::with('bar')->where_something(1) 时，Laravel 将首先加载 Foo，然后，基于 Foo.bar_id，它会加载Bar.它的目的是告诉 Laravel 在组合查询中预先加载模型的依赖项，从而大大提高这些模型的迭代性能.
When you use Foo::with('bar')->where_something(1), Laravel will first load the Foo and then, based on Foo.bar_id, it will load the Bar. It serves the purpose of telling Laravel to eager load dependencies of your model on a combined query, greatly improving performance of iterations on those models.
如果你不使用它，应该执行以下查询:
If you don't use it, the following queries should be executed:
SELECT * FROM foos WHERE foos.something = 1;
SELECT * FROM bars WHERE bars.id = 30;
SELECT * FROM bars WHERE bars.id = 57;
SELECT * FROM bars WHERE bars.id = 134;
SELECT * FROM bars WHERE bars.id = 1096;
另一方面，如果你使用它:
If you use it, on the other hand:
SELECT * FROM foos WHERE foos.something = 1;
SELECT * FROM bars WHERE bars.id IN (30, 57, 134, 1096); // Eager loading
当您向该 with() 添加条件时，您只是限制了这些依赖项的即时加载，而不是第一个查询.
When you add a condition to that with(), you are only constraining the eager loading of those dependencies, and not the first query.
要实现你想要的，你需要使用 ->join().
To achieve what you want, you'll need to use ->join().
C::with(array('b', 'b.a'))
->join('b', 'b.id', '=', 'c.b_id')
->join('a', 'a.id', '=', 'b.a_id')
->where('a.id', '=', $ID)
->get('c.*');
我已经包含了 with()，因为我不知道您是否需要访问 $c->b->a.如果不这样做，而您只需要 $c 数据，则可以删除 with()，因为它会不必要地查询 B 和 A.
I've included the with(), because I didn't know if you would need to access $c->b->a. If you don't, and you just need $c data, you can remove the with() since it will query for B's and A's unnecessarily.
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