Database
[{sign1:"A1",sign2:"A2"},{sign1:"A2",sign2:"A5"},{sign1:"A2",sign2:"A6"},{sign1:"A2",sign2:"A8"},{sign1:"A5",sign2:"A8"}]
Query
db.collection.aggregate([{"$match":{sign1:"A1"}},{"$graphLookup":{"from":"collection","startWith":"$sign1","connectFromField":"sign2","connectToField":"sign1","as":"path","depthField":"step"}}])