List all values of a certain field in mongodb
db.collection.distinct('x')
should give you an array of unique values for that field.
This would return an array of docs, containing just it's x value...
db.collection.find( { }, { x: 1, y: 0, _id:0 })
Notice: My answer is a fork from the original answer.
Before any "thumbs up" here, "thumbs up" the accepted answer :).
db.collection.distinct("NameOfTheField")
Finds the distinct values for a specified field across a single collection or view and returns the results in an array.
Reference: https://docs.mongodb.com/manual/reference/method/db.collection.distinct/