How do I query for item with non-empty array with mongoid?

This query search if exists any object in the related_question_ids[0] field

Using js shell

db.questions.find("related_question_ids.0": {exists => true} )

Using mongoid

Mongoid::Criteria.new(Question).where(:"related_question_ids.0".exists => true)

You can search greater any size

Mongoid::Criteria.new(Question).where(:"related_question_ids.3".exists =>true)

This solve your problem

You can use the $size operator to query by array sizes. Consider the following example using the JS shell:

> db.foo.drop()
> db.foo.insert({_id: 1, x:[1,2]});
> db.foo.insert({_id: 2, x:[]});
> db.foo.insert({_id: 3, x:3});

> db.foo.find({x: {$size: 0}})
{ "_id" : 2, "x" : [ ] }

> db.foo.find({x: {$size: 1}})

> db.foo.find({x: {$size: 2}})
{ "_id" : 1, "x" : [ 1, 2 ] }

> db.foo.find({x: {$not: {$size: 2}}})
{ "_id" : 2, "x" : [ ] }
{ "_id" : 3, "x" : 3 }

> db.foo.find({x: {$not: {$size: 0}}})
{ "_id" : 1, "x" : [ 1, 2 ] }
{ "_id" : 3, "x" : 3 }

I'm not familiar with Mongoid, but I found an example using $size in this documentation.

Two caveats with $size are that it cannot utilize an index (other parts of the query certainly can) and it cannot be used in range queries. If you don't mind the additional bookkeeping, a viable option is to store the array's size in a separate field (likely indexed) and query on that any way you like.

Another way to do this is to use the .nin (Not IN) form of the query:

Mongoid::Criteria.new(Question).where(:related_question_ids.nin => [nil,[]])

This will only return a Question where the related_question_ids is no nil and not an empty array.

Conversely, you can define :related_question_ids to have a default value (:default => []) and then you only need to query for .ne (Not Equal), like so:

Mongoid::Criteria.new(Question).where(:related_question_ids.ne => [])

Either should work.