如何在6个MongoDB集合中找到两个随机文档？
让我们首先创建一个集合并向其中添加一些文档
> db.twoRandomDocumentDemo.insertOne({"StudentId":10});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9ec9aad628fa4220163b87")
}
> db.twoRandomDocumentDemo.insertOne({"StudentId":100});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9ec9add628fa4220163b88")
}
> db.twoRandomDocumentDemo.insertOne({"StudentId":45});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9ec9b0d628fa4220163b89")
}
> db.twoRandomDocumentDemo.insertOne({"StudentId":55});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9ec9b3d628fa4220163b8a")
}
> db.twoRandomDocumentDemo.insertOne({"StudentId":5});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9ec9b7d628fa4220163b8b")
}
> db.twoRandomDocumentDemo.insertOne({"StudentId":7});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9ec9bad628fa4220163b8c")
}以下是在find()方法的帮助下显示集合中所有文档的查询
> db.twoRandomDocumentDemo.find();
这将产生以下输出
{ "_id" : ObjectId("5c9ec9aad628fa4220163b87"), "StudentId" : 10 }
{ "_id" : ObjectId("5c9ec9add628fa4220163b88"), "StudentId" : 100 }
{ "_id" : ObjectId("5c9ec9b0d628fa4220163b89"), "StudentId" : 45 }
{ "_id" : ObjectId("5c9ec9b3d628fa4220163b8a"), "StudentId" : 55 }
{ "_id" : ObjectId("5c9ec9b7d628fa4220163b8b"), "StudentId" : 5 }
{ "_id" : ObjectId("5c9ec9bad628fa4220163b8c"), "StudentId" : 7 }以下是从6个随机文档中查询2个的查询。将大小设置为2，因为我们只需要2个文档。
> db.twoRandomDocumentDemo.aggregate([{$sample: {size: 2}}]);这将产生以下输出
{ "_id" : ObjectId("5c9ec9b3d628fa4220163b8a"), "StudentId" : 55 }
{ "_id" : ObjectId("5c9ec9aad628fa4220163b87"), "StudentId" : 10 }这是第二种情况，当您再次运行上述查询以获取不同的文档时
> db.twoRandomDocumentDemo.aggregate([{$sample: {size: 2}}]);这将产生以下输出
{ "_id" : ObjectId("5c9ec9add628fa4220163b88"), "StudentId" : 100 }
{ "_id" : ObjectId("5c9ec9b7d628fa4220163b8b"), "StudentId" : 5 }