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db.collection.aggregate([{$match:{_f:"611531d23039d93be3bf2e2a4"}},{$lookup:{from:"collection",let:{f:"$_f",t:"$_t"},pipeline:[{$match:{$expr:{$and:[{$eq:["$_t","$$f"]},{$eq:["$_f","$$t"]}]}}},{$project:{_id:1}}],as:"hasCouple"}},{$match:{$expr:{$gt:[{$size:"$hasCouple"},0]}}},{$set:{hasCouple:{$first:"$hasCouple._id"}}}])