db={"worspace":[{"_id":1,"admins":["a1","a2"]},{"_id":2,"admins":["a3","a4"]},{"_id":3,"admins":["a3","a5"]}],"users":[{"_id":"a1","sku":"almonds","description":"product 1","instock":120},{"_id":"a2","sku":"bread","description":"product 2","instock":80},{"_id":"a3","sku":"cashews","description":"product 3","instock":60},{"_id":"a6","sku":"pecans","description":"product 4","instock":70},{"_id":"a7","sku":null,"description":"Incomplete"}]}

db.users.aggregate([{$lookup:{from:"worspace",localField:"_id",foreignField:"admins",as:"workspaces"}},{$match:{workspaces:{$ne:[]}}}])