mongodb聚合与$ project有条件地排除字段 [英] mongodb aggregation with $project to conditionally exclude a field

问题描述

我想从mongo db aggergtion管道中排除某个字段，在阅读文档后，我认为我需要指定1或0来保留该字段(或不保留该字段)(参见解决方案

从mongoDB 3.6开始，您可以使用http://docs.mongodb.org/manual/reference/operator/aggregation/project/)

So I tried the following (using node.js mongoose, but the syntax is quite the same as plain mongo):

aggregate.match({ date: { $gte : today } });
aggregate.sort('-date');
aggregate.group({
    _id: '$name',
    date: { $first: '$date' },
    user: { $first: '$user' },
    app: { $first: '$app' }
});
aggregate.project({
    _id: 1,
    last: { $cond: [ { $eq : [ '$_id', 'undo' ] }, '$date', 0 ] },
    user: { $cond: [ { $eq : [ '$_id', 'undo' ] }, '$user', 0 ] },
    app: { $cond: [ { $eq : [ '$_id', 'undo' ] }, '$app', 0 ] }
});

But doing that, I end with documents like this:

[ 
  {
    _id: 'devices',
    user: 0,
    app: 0,
    last: 0
  },
  { 
    _id: 'undo',
    user: 'test',
    app: 'truc',
    last: Mon Jan 26 2015 16:21:53 GMT+0100 (CET)
  }
]

I want the user, app and date to appear only when _id is undo.

How to achieve it?

Thanks!

解决方案

Starting in mongoDB 3.6, you can use the REMOVE variable to exclude fields conditionally.

In your particular case, the project stage should look like this:

aggregate.project({
    _id: 1,
    last: { $cond: [ { $eq : [ '$_id', 'undo' ] }, '$date', '$$REMOVE' ] },
    user: { $cond: [ { $eq : [ '$_id', 'undo' ] }, '$user', '$$REMOVE' ] },
    app: { $cond: [ { $eq : [ '$_id', 'undo' ] }, '$app', '$$REMOVE' ] }
});

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