Mongodb内嵌文档的查询

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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"                }        ]}

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