如何在Mongo DB中的多个文档中查找所需的值

Question

I have multiple documents in the collection then I need to find the particular value inside a single document. Is there any way, we can do this? Thanks for any help.

This is the following document that I need to fetch some specific values from the whole collection. Its having four collections and we need to find a single value for a particular document inside the collection.

{
"A":{
"A1":"Value A1"
},
"B":{
"B1":"Value B1"
},

"C":{
"C1":"Value C1"
},

"D":{
"D1":"Value D1"
}}

I want to get Value of D1 in the above collection. Is there any way to find it in as single query.

Answer 1

I'm confused a little bit, but maybe you want projections . You would do something like this (using the java driver ):

findIterable = collection.find(eq("_id", "some id")).projection(include("D.D1"));

This way you get only one record matching the query for which only id and D.D1 are returned. You can of course change the query to get more records with only specified fields returned.

If you mean something else, please clarify.

EDIT : I can see the answer above, so think about using exists operator as well, something like:

findIterable = collection.find(exists("D1")).projection(include("D1"));

This way you get only records having this field and only this field is returned for them (and their ids).