CODEWITHSUNDEEP
javamongodbmongodb-java
teja
teja

Reputation: 127

Retrieve only the queried element in an object array in MongoDB using java

Suppose we have the following documents in a MongoDB collection: 

{  
   "_id":ObjectId("562e7c594c12942f08fe4192"),
   "shapes":[  
      {  
         "shape":"square",
         "color":"blue"
      },
      {  
         "shape":"circle",
         "color":"red"
      }
   ]
},
{  
   "_id":ObjectId("562e7c594c12942f08fe4193"),
   "shapes":[  
      {  
         "shape":"square",
         "color":"black"
      },
      {  
         "shape":"circle",
         "color":"green"
      }
   ]
}

And the MongoDB query is

db.test.find({"shapes.color": "red"}, {_id: 0, 'shapes.$': 1});

Can someone tell me how to write it in Java?

I am using: 

List<BasicDBObject> obj = new ArrayList<>();
obj1.add(new BasicDBObject("shapes.color", "red"));
List<BasicDBObject> obj1 = new ArrayList<>();
obj2.add(new BasicDBObject("shapes.$", "1"));

BasicDBObject parameters1 = new BasicDBObject();
parameters1.put("$and", obj1);

DBCursor cursor = table.find(parameters1,obj2).limit(500);

and I am not getting anything.

Upvotes: 1

Views: 1386

Answers (1)

glytching
glytching

Reputation: 47875

The syntax of the Mongo Shell find function is:

db.collection.find(query, projection)

query document Optional. Specifies selection filter using query operators. To return all documents in a collection, omit this parameter or pass an empty document ({}).

projection document Optional. Specifies the fields to return in the documents that match the query filter.

When translating this for execution by the Mongo Java driver you need to construct separate BasicDBObject instances for;

  • the query
  • the projection

Here's an example:

MongoCollection<Document> table = ...;

// {"shapes.color": "red"}
BasicDBObject query = new BasicDBObject("shapes.color", "red");

// {_id: 0, 'shapes.$': 1}
BasicDBObject projection = new BasicDBObject("shapes.$", "1").append("_id", 0);

FindIterable<Document> documents = table
    // assign the query
    .find(query)
    // assign the projection
    .projection(projection);

System.out.println(documents.first().toJson());

Given the sample documents included in your question the above code will print out:

{
  "shapes": [
    {
      "shape": "circle",
      "color": "red"
    }
  ]
}

This is identical to the output from db.test.find({"shapes.color": "red"}, {_id: 0, 'shapes.$': 1});.

Upvotes: 3

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