如何使用MongoDB $ elemMatch获得所有匹配项？

Question

I have two objects:

{
  genre: ['music', 'movie']
}

and

{
 genre: ['movie', 'music']
}

and my query is:

db.test.find({genre :{ $elemMatch:{ $in : ['movie']}}})

and it only gives me the second object. Why? I want to get all the docs that contain a specific genre in their arrays no matter where in the array. How can I do this?

Answer 1

You need neither $elemMatch nor $in in this case. A simple field:value will match documents where field is an array and any one of the values in that array is value . That means

db.test.find({genre :'movie'});

will suffice.

The array query operators are required in more complex situations.

• $in is needed when you have a list of possible values and want documents where any of them is found (so db.test.find({genre : { $in:['movie']} }); would work, but would be needlessly convoluted)
• $all works like $in but requires that all provided elements are in the array
• $elemMatch is a bit more complex. It is required when you want to use multiple operator-conditions (like $gt or $lt) but want only those documents where one array entry matches all the conditions. Without the $elemMatch operator, you get results where each condition is met by at least one array entry, but not necessarily all by the same entry.

Answer 2

Remember db.test.find() in general returns you cursor object. You can access all doc as follows:

entry = db.test.find({genre :{ $elemMatch:{ $in : ['movie']}}})
for doc in entry:
    print(doc)