增加关联延迟预载入（仅针对数据集对象）

library/think/Collection.php

@@ -16,6 +16,7 @@ use ArrayIterator;
 use Countable;
 use IteratorAggregate;
 use JsonSerializable;
+use think\Model;
 
 class Collection implements ArrayAccess, Countable, IteratorAggregate, JsonSerializable
 {
@@ -352,6 +353,21 @@ class Collection implements ArrayAccess, Countable, IteratorAggregate, JsonSeria
         return json_encode($this->toArray(), $options);
     }
 
+    /**
+     * 延迟预载入关联查询
+     * @access public
+     * @param mixed $relation 关联
+     * @return $this
+     */
+    public function load($relation)
+    {
+        $item = current($this->items);
+        if ($item instanceof Model) {
+            $item->eagerlyResultSet($this->items, $relation);
+        }
+        return $this;
+    }
+
     public function __toString()
     {
         return $this->toJson();