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In this question, Jeff the Bear explained how to search documents with an array

contains 'tag1'
contains ['tag1','tag2'],
contains any of ['tag3', 'tag4']

But how should I do if I want to search documents with an array which is part of another array? 

post1.tags = ['tag1']
post2.tags = ['tag1','tag3']
post3.tags = ['tag2','tag4']
post4.tags = ['tag1','tag2','tag3','tag4']

I want to get post1 and post3 because they have tags 

contained in ['tag1', 'tag2', 'tag4']

I don't want to get post2 and post4, because tag3 doesn't exist in ['tag1', 'tag2', 'tag4']

In other words, select posts that all elements in its tags array can be found in another conditional array

share|improve this question
    
What do you mean by "part of another array" should tags contain all of what your looking for or just some of what your looking for? –  Sammaye Mar 26 '13 at 11:17
    
Either {post1.tags = ['tag1']} or {post2.tags = ['tag1', 'tag3']} is a part of array ['tag1','tag3']. –  Orion Chang Mar 26 '13 at 11:19
    
Provided you can garauntee the order you could do: db.col.find({tags:[tag1,tag2]}) but it requires a good order –  Sammaye Mar 26 '13 at 11:20
    
Ok I see your edit, it seems like: db.col.find({tags: {$in:[tag1,tag2]}}) should work –  Sammaye Mar 26 '13 at 11:33
    
db.col.find({tags: {$in:[tag1,tag2]}}) will search any posts with tags contain "tag1" or "tag2". I may get some posts with tags ["tag1","tag3"] or ["tag2","tag4"], and they aren't the posts I'm looking for. In this case, I only want posts with tags["tag1"],["tag2"],or ["tag1","tag2"] –  Orion Chang Mar 26 '13 at 11:45

1 Answer 1

up vote 2 down vote accepted

You can use aggregation framework to do this. Given the following data

> db.post.find()
{ "_id" : 1, "tags" : [ "tag1" ] }
{ "_id" : 2, "tags" : [ "tag1", "tag3" ] }
{ "_id" : 3, "tags" : [ "tag2", "tag4" ] }
{ "_id" : 4, "tags" : [ "tag1", "tag2", "tag3", "tag4" ] }

the aggregation query 

db.post.aggregate({
  $project: {
    _id: 1,
    tags: 1,
    killFlag: {
      $const: [true, false]
    }
  }
}, {
  $unwind: "$tags"
}, {
  $unwind: "$killFlag"
}, {
  $match: {
    $nor: [{
        tags: {
          $in: ['tag1', 'tag2', 'tag4']
        },
        killFlag: true
      }
    ]
  }
}, {
  $group: {
    _id: "$_id",
    tags: {
      $addToSet: "$tags"
    },
    killFlag: {
      $max: "$killFlag"
    }
  }
}, {
  $match: {
    killFlag: false
  }
}, {
  $project: {
    _id: 1,
    tags: 1
  }
})

would give you

{
  "result": [{
      "_id": 3,
      "tags": [
          "tag4",
          "tag2"
      ]
    }, {
      "_id": 1,
      "tags": [
          "tag1"
      ]
    }
  ],
  "ok": 1
}
share|improve this answer
    
It works very well, thank you very much. –  Orion Chang Mar 31 '13 at 14:33

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