reduce mongo db query to pure array

Question

when i run db.abhishek.em.find({}) I have

{ "_id" : ObjectId("5ac62d35b075e574b3e7eeaa"), "name" : "first", "employed" : true }
{ "_id" : ObjectId("5ac62d3fb075e574b3e7eeab"), "name" : "second", "employed" : true }
{ "_id" : ObjectId("5ac62d4eb075e574b3e7eeac"), "name" : "third", "employed" : false }

I want to reduce this result into an array of simple object ids like this by adding or chaining something to find function , something like db.a.b.find({employed:true}).somefunction() which can return the below array I want to use the command nested with $in in a bigger query to achieve some sort of relational querying

[
ObjectId("5ac62d35b075e574b3e7eeaa"),
ObjectId("5ac62d3fb075e574b3e7eeab")
]

-----------------EDIT----------------------

For an example case I want to get employed employees by running

db.abhishek.another.find({id:{$in:db.abhishek.em.find({employed:true},{_id:1}).toArray()}})

or something similiar as this command is not working

The db.a.another collection is

{ "_id" : ObjectId("5ac63de1b075e574b3e7eead"), "id" : ObjectId("5ac62d35b075e574b3e7eeaa"), "name" : "Lets say person 1" }
{ "_id" : ObjectId("5ac63df7b075e574b3e7eeae"), "id" : ObjectId("5ac62d3fb075e574b3e7eeab"), "name" : "Lets say person 2" }
{ "_id" : ObjectId("5ac63e06b075e574b3e7eeaf"), "id" : ObjectId("5ac62d4eb075e574b3e7eeac"), "name" : "Lets say person 3" }

-----------------EDIT---------------------- Solved see my answer below

I want to use the command nested with $in in a bigger query to achieve some sort of relational querying -> What do you really want? — Jonas Wilms
– Jonas Wilms, Commented Apr 5, 2018 at 14:55
toArray gives me the complete array of objects (each bieng the complete object), i just want an array of Object ids and no further nesting to use it within $in — Abhishek Mathur
– Abhishek Mathur, Commented Apr 5, 2018 at 14:55
@jonas-w I basically want to use something like db.a.c.find({_id:{$in:db.a.b.find{employed:true}.somemethod_thatworkswith$in}}) — Abhishek Mathur
– Abhishek Mathur, Commented Apr 5, 2018 at 14:57

Randy Casburn · Accepted Answer · 2018-04-05 14:58:54Z

2

Use MongoDB to only provide those fields from the model. In MongoDB terms this is called Projection. There is a MongoDB method .project() that you can chain onto your query like this:

db.abhishek.em.find({}).project({});

that should do it. If you want to explicit exclude fields, you do:

db.abhishek.em.find({}).project({name:0, employed:0});

then, finally, to get the output in array form do:

db.abhishek.em.find({}).project({name:0, employed:0}).toArray();

Reference here:

https://docs.mongodb.com/manual/tutorial/project-fields-from-query-results/

answered Apr 5, 2018 at 14:58

Randy Casburn

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2 Comments

Abhishek Mathur Over a year ago

it cannot find project method,what could have gone wrong?

Randy Casburn Over a year ago

The implementations differ per platform. The example I gave was for MongoDB from NodeJS. What platform are you using? In the reference I provided above, click the tab at the top of the page that corresponds do your platform.

Abhishek Mathur · Accepted Answer · 2018-04-05 15:22:55Z

0

Thanks everyone for providing directions and suggestions to move forward, I was able to successfully do some sort of relational based query using the following

db.abhishek.another.find({id:{$in:db.abhishek.em.find({employed:true},{_id:1}).map(function(e) {return e._id})}},{name:1,_id:0})

It gave this as output

{ "name" : "Lets say person 1" }
{ "name" : "Lets say person 2" }

This query took all records from em collection with employed set to true,form its array and then pass it to $in operator on query for "another" collection to give output

toArray method used to convert objects nested in array hence failing $in operator

Thanks @veeram for telling about selection of fields

answered Apr 5, 2018 at 15:22

Abhishek Mathur

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