How do I query an array of dictionaries in MongoDB?

问题

I have an array of dictionaries on which I have to make queries. The queries will be like when "name" is "a" then "value" should be "2".

{
    "t": "m",
    "y": "n",
    "A":[ 
            {
             "name": "x",
             "value": "1"
            },
            {
             "name": "y",
             "value": "2"
            },
            {
             "name": "z",
             "value": "1"
            }
        ]
}

In the above, I want to know what are the records whose "value" is "1" when "name" is x. I also need to make queries like, where "name" is "x" then value should be "2" and "name" is "y" then "value" should be "1"

回答1:

You have to use $elemMatch to query embedded documents in an array if you want to query with multiple fields of embedded document. So your query should be like this:

db.collection.find( {
  "A": { $elemMatch: { name: "x", value: "1" } }
})

If you want query documents which have (name:"x", value:"1") or (name:"y", value:"2") in same query, you can use $orwith elemMatch like this:

db.collection.find( {
  $or: [
    { "A": { $elemMatch: { name: "x", value: "1" } } },
    { "A": { $elemMatch: { name: "y", value: "2" } } }
  ]  
})

If you want query documents which have (name:"x", value:"1") and (name:"y", value:"2") in same query, you can use $andwith elemMatch like this:

db.collection.find( {
  $and: [
    { "A": { $elemMatch: { name: "x", value: "1" } } },
    { "A": { $elemMatch: { name: "y", value: "2" } } }
  ]  
})

回答2:

I am using it like this and it's working.

db.collection.find(
                  {
                  $and:[
                       {"A.name":"x", "A.value": "2"},
                       {"A.name":"y", "A.value": "3"}, 
                       {"t": "m"}
                       ]
                  }

The above will give all records where "t" is "m" and where dictionary with name "x" has value "2" and dictionary with name "y" has value "3".

回答3:

One approach is to use $unwind and $match to get the desired result $unwind will unwind the array A, and then using $match we can pick out the matching documents.

Sample query

db.collectionname.aggregate([
      {$unwind:"$A"}, 
      {$match:{"A.name":"x", "A.value":"1"}}
])

Sample result

{ "_id" : ObjectId("59c9ee65f1287059437fa3ac"), "t" : "m", "y" : "n", 
   "A" : { "name" : "x", "value" : "1" } }

来源：https://stackoverflow.com/questions/46418832/how-do-i-query-an-array-of-dictionaries-in-mongodb