mongo在数组字段中找到var的位置

Question

I have mongo data that looks like:

{
    "name": "test1",
    "source": ["source1"]
},
{ 
    "name": "test2",
    "source": ["source1", "source2"]
}

I need to do a find for a document where "source1" is in the source array. I've tried to get $elemMatch to work as:

db.myCollection.find({source: {$elemMatch: {$eq:['source1']}}}) also tried db.myCollection.find({source: {$elemMatch: {$eq:{source: "source1"}}}})

Neither return any documents.

I must be using the $elemMatch wrong, or else it may be the wrong thing to use. How can I do a find where an array field in a document contains a specific element?

Answer 1

Use the below syntax when you want to search for simple elements and not sub-documents.

db.mycollection.find({"source":"source1"}})

This works because the elements in the array are not documents, but literals. The query condition is applied on each literal. If a matching literal is found, that particular document is returned.

If you do want to use $elemMatch , which is not necessary here, the syntax would be:

db.mycollection.find({"source":{$elemMatch:{$eq:"source1"}}})

The query that you tried:

db.myCollection.find({source: {$elemMatch: {$eq:{source: "source1"}}}})

would work if the source array were to have sub documents in them, such as:

{ 
    "name": "test2",
    "source": [{"source":"source1"}]
}

Answer 2

You do not need $elemMatch here. If your field is an array ( source: [1, ..., 4] ) and all you need is to find whether one element exist there, all you need is a simple find.

db.mycollection.find({source: 'source1'}})

Also note that you can put an index on the field source and it will work just fine.