我有以下文件:
{ _id : 21353456, username : "xy", text : "asdf", comments : [ { username : "User1", text : "hi", }, { username : "User2", text : "hi1", }, { username : "User3", text : "hi2", }, { username : "User4", text : "hi3", } ] }现在,我想获取带有聚合和项目的用户名,文本和注释.另外,如果comments数组包含带有"User1"的用户名,我还需要一个布尔值.我有这个,但是不起作用.
Now I want to get the username, text and comments with aggregation and project. In addition I also want a boolean if the comments array contains an username with "User1". I have this, but it doesn't work.
db.posttest.aggregate( [ { $project: { username: 1, text: 1, comments : 1, hasComment: { $eq: [ "comments.$.username", "User1" ] }, _id: 0 } } ] )推荐答案
要实现此目的,您需要先unwind注释,然后使用带有一些技巧的group.如果要省略_id,则还需要做一个简单的项目.这是完整的聚合管道:
To achieve this you would need to first unwind the comments, and then use a group with a little trick. If you want to omit the _id, then you would also need to do a simple project. Here is the full aggregation pipeline:
db.posttest.aggregate([ { $unwind : "$comments" }, { $group : { _id : "$_id", username : { $first : "$username" }, text : { $first : "$text" }, comments : { $push : "$comments" }, hasComments : { $max : { $eq : [ "$comments.username", "User1" ] } } }}, { $project : { _id : false } } ])解释如下.
首先,我们需要摆脱一个数组(comments).为此,我们释放记录.它给了我们四个记录:
First, we need to get rid of an array (comments). To do this we unwind the record; it gives us four records:
{ "_id" : 21353456, "username" : "xy", "text" : "asdf", "comments" : { "username" : "User1", "text" : "hi" } }, { "_id" : 21353456, "username" : "xy", "text" : "asdf", "comments" : { "username" : "User2", "text" : "hi1" } }, { "_id" : 21353456, "username" : "xy", "text" : "asdf", "comments" : { "username" : "User3", "text" : "hi2" } }, { "_id" : 21353456, "username" : "xy", "text" : "asdf", "comments" : { "username" : "User4", "text" : "hi3" } }现在,我们可以将所有记录归为一组,从而对每个字段应用一个功能.首先,我们需要提供标准,即分组依据"字段(或一组字段).在我们的例子中,它只是ID:_id: "$_id".
Now we can group all the records into one applying a function to each field. First, we need to give criteria, the 'group by' field (or set of fields). In our case, it is simply the id: _id: "$_id".
然后,对于每个字段,我们需要决定如何将其包含到结果记录中.我们只有几个字段:username,text和comments.对于每四个记录,用户名和文本都是相同的,因此我们可以轻松地选择它们中的任何一个,即$first或$last.
Then, for each field, we need to make a decision on how to include it into the resulting record. We have few fields: username, text, and comments. For each four records the username and text are the same, so we can easily pick any of them, namely $first or $last.
comments是不同的.我们想保留所有这些,以便我们每个人都返回$push.
comments, however, are different. We want to preserve all of them so that we $push each one back.
这里的hasComments有点棘手:我们需要检查是否至少有一个comment.username包含用户名.我们可以在这里使用$eq: [...],它将为我们提供一些数组,例如[true, false, false, false]或[false, false, true, false].我们需要选择将哪个值记入结果记录中.在这种情况下,我们不能使用$first或$last.但是,$max将为我们提供适当的结果.
The hasComments is a bit tricky here: we need to check if at least one comment.username contains the username. We can use $eq: [...] here, it will give us some array, e.g. [true, false, false, false] or [false, false, true, false]. We would need to pick which value goes into the resulting record. In this case, we can use neither $first nor $last. However, $max will give us an appropriate result.
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