MongoDB : querying documents with two equal fields, $match and $eq

Question

What is the best way to return all documents in a collection if I want document.a == document.b?

I\'ve tried

db.collection.aggregate([ { $match: { $         


        

Answer 1

Basically, you are trying to perform a self join. An operation not supported by MongoDB.

Concerning the $eq operator, as you guessed:

• By design, the $eq comparison query operator match a field against a value.
• But the $eq comparison aggregation operator compare the value of two expressions.

I don't know any other way to perform what you need than using an extra $project step as you suggested.

Please note this is not significantly more expensive as, anyway, your query cannot use any index and MongoDB will do a full scan.

Answer 2

If I understood your question right you want those documents that have same values in field1 and field2.

For this try

db.coll.find({$where: function() { return this.field1 == this.field2 } } );

or more compact

db.coll.find({ $where : "this.field1 == this.field2" } );