db={"chat":[{_id:1,lender_id:10,borrower_id:20,message:"Message from lender"},{_id:2,lender_id:20,borrower_id:10,message:"Response to lender"},{_id:3,lender_id:10,borrower_id:30,message:"Message to other user"}]}

db.chat.aggregate([{$match:{$or:[{lender_id:10},{borrower_id:10}]}},{$group:{_id:null,differentChats:{$addToSet:{$cond:[{$gt:["$lender_id","$borrower_id"]},["$lender_id","$borrower_id"],["$borrower_id","$lender_id"]]}}}},{$project:{differentChats:{$size:"$differentChats"}}}])