Error in Projection using find() method

Question

I am total newbie to MongoDB. I was trying out basic stuff in mongo, when I encountered a problem. I searched for it, but not able to find any satisfactory answer.

I have a very simple collection named "users" having names and age of some persons. Below is output of db.users.find()

{ "_id" : ObjectId("566acc0442fea953b8d94a7e"), "name" : "gabriel", "age" : 22 }
{ "_id" : ObjectId("566acc0442fea953b8d94a7f"), "name" : "andy", "age" : 10 }
{ "_id" : ObjectId("566acc1342fea953b8d94a80"), "name" : "henry", "age" : 27 }
{ "_id" : ObjectId("566acc1342fea953b8d94a81"), "name" : "william", "age" : 19 }
{ "_id" : ObjectId("566acc3242fea953b8d94a82"), "name" : "sandra", "age" : 20 }
{ "_id" : ObjectId("566acc3242fea953b8d94a83"), "name" : "tom", "age" : 24 }

Now, I am trying to apply different projection on it. First two are:

db.users.find({}, {"name":1, "age":1})

{ "_id" : ObjectId("566acc0442fea953b8d94a7e"), "name" : "gabriel", "age" : 22 }
{ "_id" : ObjectId("566acc0442fea953b8d94a7f"), "name" : "andy", "age" : 10 }
{ "_id" : ObjectId("566acc1342fea953b8d94a80"), "name" : "henry", "age" : 27 }
{ "_id" : ObjectId("566acc1342fea953b8d94a81"), "name" : "william", "age" : 19 }
{ "_id" : ObjectId("566acc3242fea953b8d94a82"), "name" : "sandra", "age" : 20 }
{ "_id" : ObjectId("566acc3242fea953b8d94a83"), "name" : "tom", "age" : 24 }

db.users.find({}, {"name":0, "age":0})

{ "_id" : ObjectId("566acc0442fea953b8d94a7e") }
{ "_id" : ObjectId("566acc0442fea953b8d94a7f") }
{ "_id" : ObjectId("566acc1342fea953b8d94a80") }
{ "_id" : ObjectId("566acc1342fea953b8d94a81") }
{ "_id" : ObjectId("566acc3242fea953b8d94a82") }
{ "_id" : ObjectId("566acc3242fea953b8d94a83") }

are working just fine, but

db.users.find({}, {"name":0, "age":1})

Error: error: { "$err" : "Can't canonicalize query: BadValue Projection cannot have a mix of inclusion and exclusion.", "code" : 17287 }

is failing and giving error as shown above. I searched that problem may arise if there is conflicting conditions in projection, but I don't think there is something like that in my method call. Is there some rule like value of fields in find method should be either all ZERO or all ONE, but cannot be both. Please help.

Answer 1

If you want your query to return only the age, your projection must include only the fields you want to have. Not the one you don't want:

db.projection.find({}, {_id:0, age:1})

Exception for _id, you can specify to not include it.

Result

{
  "age": 22
}

From the documentation:

A projection cannot contain both include and exclude specifications, except for the exclusion of the _id field. In projections that explicitly include fields, the _id field is the only field that you can explicitly exclude.