Want to update the field_a1 of collection a with the value of filed_b1 from collection b using connection key field_a2 = filed_b2 with collection a have field flag is set.
Solution:(using javascript in Mongo Shell)
db.a.find({flag:{$exists: true}}).forEach( function (item) { item.field_a1 = db.b.findOne({field_b2: item.field_a2}).field_b1; db.a.save(item); } )
Enjoy it!