在 MongoDB 中，如何索引数组中的嵌入对象字段？
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In MongoDB how do you index an embedded object fields in an array?
提问by Imran
The mongodb documentation for multikeys gives an example of querying embedded object fields in an array:
多键的 mongodb 文档给出了一个查询数组中嵌入对象字段的示例：
http://www.mongodb.org/display/DOCS/Multikeys
http://www.mongodb.org/display/DOCS/Multikeys
But there's no explanation on how you create an index for that situation. Creating an index on the array doesn't seem to work (using the explain mechanism you can see the index isn't use).
但是没有解释如何为这种情况创建索引。在数组上创建索引似乎不起作用（使用解释机制，您可以看到未使用索引）。
Additional details:
额外细节：
> // find posts where julie commented
> db.posts.find( { "comments.author" : "julie" } )
{"title" : "How the west was won",
"comments" : [{"text" : "great!" , "author" : "sam"},
{"text" : "ok" , "author" : "julie"}],
"_id" : "497ce79f1ca9ca6d3efca325"}
If you do db.articles.ensureIndex( { comments : 1 } )it won't index the subfields of the comments objects but rather only the comments object itself.
如果你这样做，db.articles.ensureIndex( { comments : 1 } )它不会索引评论对象的子字段，而只会索引评论对象本身。
So the following would use the index:
因此，以下将使用索引：
> db.posts.find( {comments : { "author" : "julie", "text" : "ok" } } )
Because it's search on the comments objects
因为它是在评论对象上搜索
But the following wouldn't use the index:
但以下不会使用索引：
> db.posts.find( { "comments.author" : "julie" } )
So how do you get mongodb to index for the second case?
那么如何让 mongodb 为第二种情况建立索引呢？
回答by Remon van Vliet
You can create the following index :
您可以创建以下索引：
db.posts.ensureIndex({"comments.author" : 1})
This will index only the author field of the embedded documents. Note that the index will be used for
这将仅索引嵌入文档的作者字段。请注意，该索引将用于
db.posts.find( { "comments.author" : "julie" } )
As well as
也
db.posts.find( { comments: {$elemMatch: {author : "julie" }}} )
回答by japrescott
you create the index as if you would with a "normal" field;
您创建索引就像使用“普通”字段一样；
db.[collection].ensureIndex( { [yourArrayField] : 1 } )
