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

问题

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']

回答1:

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;

回答2:

You can do this with distinct:

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

Output:

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

来源：https://stackoverflow.com/questions/27606591/how-to-retrieve-partial-objects-from-object-array-in-a-field-in-mongodb