how to retrieve partial objects from object array in a field in mongodb

Question

I have a mongoose schema like -

db.foo.insert({one:'a',friends:[{two:'b',three:'c'},{two:'d',three:'e'},{two:'f',three:'G'}]})

now what i want is two retrieve only the 'two' part of friends array that is I want to find an array of all the values of two in each object in friends array Is such a projection possible in mongodb where in the output looks like -

['b','d','f']

@JohnnyHK I just edited the question to include the expected answer. — Harshit Laddha
– Harshit Laddha, Commented Dec 22, 2014 at 16:44
No, they are unique values. I am inserting that way only. Sorry forgot to mention that earlier — Harshit Laddha
– Harshit Laddha, Commented Dec 22, 2014 at 16:50

Disposer · Accepted Answer · 2014-12-22 17:09:43Z

4

aggregate is your answer

db.foo.aggregate({"$project" : {"two" : "$friends.two"}}).result

there is another way to do that (getting distinct values)

db.foo.aggregate([      
    {'$project': {  
                    union:{$setUnion:["$friends.two"]}
                 }
    }
]).result;

edited Dec 22, 2014 at 17:09

answered Dec 22, 2014 at 16:42

Disposer

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JohnnyHK · Accepted Answer · 2014-12-22 16:50:53Z

4

You can do this with distinct:

 db.foo.distinct('friends.two')

Output:

[
  "b",
  "d",
  "f"
]

answered Dec 22, 2014 at 16:50

JohnnyHK

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1 Comment

Harshit Laddha Over a year ago

It says Error:Distinct is not implemented. in the try.mongodb.org. but worked fine on my desktop. Thanks for the answer

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