Mongodb内嵌文档的查询
在《MongoDB权威指南》这本书中4.3.4 中有个查询文档的章节，里面用到了find和elemMatch，但是给出的例子不是很详细，所以理解起来有点困难，我们来用例子来演示下。
0.目的
为了在blog集合中找到由Joe发表的5分以上的评论
1.表结构如下
> db.blog.findOne()
{
"_id" : ObjectId("55814703e6c873d825fdcc1a"),
"content" : "...",
"conments" : [
{
"author" : "joe",
"score" : 3,
"comment" : "nice post"
},
{
"author" : "joe",
"score" : 6,
"comment" : "terrible post"
}
]
}2.构建测试样例
db.blog.insert({"content":"...", "conments":[ { "author":"joe", "score":3, "comment":"nice post" }, { "author":"joe", "score":6, "comment":"terrible post" } ] })
db.blog.insert({"content":"...", "conments":[ { "author":"joee", "score":3, "comment":"nice post" }, { "author":"joee", "score":6, "comment":"terrible post" } ] })
db.blog.insert({"content":"...", "conments":[ { "author":"joe", "score":3, "comment":"nice post" }, { "author":"joee", "score":6, "comment":"terrible post" } ] })
db.blog.insert({"content":"...", "conments":[ { "author":"joee", "score":3, "comment":"nice post" }, { "author":"joe", "score":6, "comment":"terrible post" } ] })> db.blog.find()
{
"_id" : ObjectId("55814703e6c873d825fdcc1a"),
"content" : "...",
"conments" : [
{
"author" : "joe",
"score" : 3,
"comment" : "nice post"
},
{
"author" : "joe",
"score" : 6,
"comment" : "terrible post"
}
]
}
{
"_id" : ObjectId("55814711e6c873d825fdcc1b"),
"content" : "...",
"conments" : [
{
"author" : "joee",
"score" : 3,
"comment" : "nice post"
},
{
"author" : "joee",
"score" : 6,
"comment" : "terrible post"
}
]
}
{
"_id" : ObjectId("55814719e6c873d825fdcc1c"),
"content" : "...",
"conments" : [
{
"author" : "joe",
"score" : 3,
"comment" : "nice post"
},
{
"author" : "joee",
"score" : 6,
"comment" : "terrible post"
}
]
}
{
"_id" : ObjectId("55814725e6c873d825fdcc1d"),
"content" : "...",
"conments" : [
{
"author" : "joee",
"score" : 3,
"comment" : "nice post"
},
{
"author" : "joe",
"score" : 6,
"comment" : "terrible post"
}
]
}3.测试1
db.blog.find({"comments" : {"author" : "joe", "score": {"$gte" : 5 }}})没有输出任何内容，所以不正确。
4.测试2
db.blog.find({“conments.author”:”joe”, “conments.score”:{“$gte”:5}})
结果如下：
db.blog.find({"conments.author":"joe", "conments.score":{"$gte":5}})
{
"_id" : ObjectId("55814703e6c873d825fdcc1a"),
"content" : "...",
"conments" : [
{
"author" : "joe",
"score" : 3,
"comment" : "nice post"
},
{
"author" : "joe",
"score" : 6,
"comment" : "terrible post"
}
]
}
{
"_id" : ObjectId("55814719e6c873d825fdcc1c"),
"content" : "...",
"conments" : [
{
"author" : "joe",
"score" : 3,
"comment" : "nice post"
},
{
"author" : "joee",
"score" : 6,
"comment" : "terrible post"
}
]
}
{
"_id" : ObjectId("55814725e6c873d825fdcc1d"),
"content" : "...",
"conments" : [
{
"author" : "joee",
"score" : 3,
"comment" : "nice post"
},
{
"author" : "joe",
"score" : 6,
"comment" : "terrible post"
}
]
}但是你会发现第二条文档并不满足条件也给find出来了。
5.正确做法
db.blog.find({"conments": { "$elemMatch" : { "author" : "joe", "score": {"$gte": 5} }}} )结果如下：
{
"_id" : ObjectId("55814703e6c873d825fdcc1a"),
"content" : "...",
"conments" : [
{
"author" : "joe",
"score" : 3,
"comment" : "nice post"
},
{
"author" : "joe",
"score" : 6,
"comment" : "terrible post"
}
]
}
{
"_id" : ObjectId("55814725e6c873d825fdcc1d"),
"content" : "...",
"conments" : [
{
"author" : "joee",
"score" : 3,
"comment" : "nice post"
},
{
"author" : "joe",
"score" : 6,
"comment" : "terrible post"
}
]
}