如何将两个数组合并为一个数组？(How can I combine two arrays in into one single array?)

 不，Laravel默认不会加载。 让我们一步一步来。 让我们忽略->schoolclasses; 一会儿。 
 
App\School::with('schoolclasses')->find(2283);
 
 这将查询数据库两次。 首先，它将使School的主键为2283.然后，它将立即查询数据库并获得所有相关的schoolclasses 。 
 
App\School::find(2283);
 
 这只会查询一次数据库。 它只会得到School 。 到目前为止还没有急切的加载。 如果您调试并跟踪数据库查询，您将看到这只会查询数据库一次，而渴望加载将查询它两次。 
 
 当你尝试通过做->schoolclasses;访问schoolclasses ->schoolclasses; ，一切都真正起作用。 那么为什么你会得到相同的结果呢？ 这有点欺骗性，但不一样。 当您尝试访问schoolclasses ，Laravel将检查它是否已经急切加载。 如果是这样，它会返回schoolclasses的集合。 没有查询。 它只是立即返回它们。 但是，如果您没有急于加载它们，Laravel将在现场查询数据库并获得schoolclasses 。 最终，对于这个特定的例子，你会得到相同的结果，但是当你查询数据库时会有所不同。 
 
 然而，这实际上是热切加载的主要好处的一个不好的例子。 
 
 急切加载的主要好处是缓解N + 1查询问题。 假设你想要5所学校和所有的课程。 没有急切的加载，这是你会做的： 
 
$schools = School::take(5)->get();

foreach ($schools as $school)
{
    $schoolclasses = $school->schoolclasses;
}
 
 对于这样一个简单的任务，总共有6个查询。 我在下面添加了注释以了解查询来自哪里： 
 
$schools = School::take(5)->get(); // First query

foreach ($schools as $school)
{
    // For each school, you are querying the database again to get its related classes.
    // 5 schools = 5 more queries
    $schoolclasses = $school->schoolclasses;
}
 
 但是，如果您急于加载所有内容，则只有两个查询： 
 
// Two queries here that fetches everything
$schools = School::with('schoolclasses')->take(5)->get();

foreach ($schools as $school)
{
    // No more queries are done to get the classes because
    // they have already been eager loaded
    $schoolclasses = $school->schoolclasses;
}

No, Laravel does not eager load by default. Lets take this step by step. Lets ignore ->schoolclasses; for a moment.
 
App\School::with('schoolclasses')->find(2283);
 
This will query the database twice. First, it will get the School with a primary key of 2283. Then, it will immediately query the database and get all the related schoolclasses.
 
App\School::find(2283);
 
This will only query the database once. It will only get the School. There is no eager loading done so far. If you debug and keep track of your database queries, you will see that this will only query the database once while eager loading will query it twice.
 
When you try to access the schoolclasses by doing ->schoolclasses;, everything actually works. So why do you get the same results? It is a bit deceptive, but it's not the same. When you try to access the schoolclasses, Laravel will check if it has already been eager loaded. If so, it will return the collection of schoolclasses. No querying is done. It just immediately returns them. However, if you did not eager load them, Laravel will query the database on the spot and get the schoolclasses. Ultimately, for this particular example, you get the same results, but when you query the database is different.
 
However, this is actually a poor example of the main benefit to eager loading.
 
The main benefit of eager loading is to alleviate the N + 1 query problem. Lets say you want to get 5 schools and all of its classes. Without eager loading, this is what you would do:
 
$schools = School::take(5)->get();

foreach ($schools as $school)
{
    $schoolclasses = $school->schoolclasses;
}
 
That is a total of 6 queries for such a simple task. I added comments below to understand where the queries are coming from:
 
$schools = School::take(5)->get(); // First query

foreach ($schools as $school)
{
    // For each school, you are querying the database again to get its related classes.
    // 5 schools = 5 more queries
    $schoolclasses = $school->schoolclasses;
}
 
However, if you eager load everything, you only have two queries:
 
// Two queries here that fetches everything
$schools = School::with('schoolclasses')->take(5)->get();

foreach ($schools as $school)
{
    // No more queries are done to get the classes because
    // they have already been eager loaded
    $schoolclasses = $school->schoolclasses;
}