javascript - How can I check field exist or not in mongo db before querying to mongodb? -

i have run dynamic query on collections in such way user enters collection name, field name , field value , have first of check whether field name supplied user exists in collection , if not have run query shown in example below.

for example: if user enters collection name user, field name type , field value article. , based on parameters have query follows:

1) if type field exists in collection user, query be:

query = user.find({type:'article'}) 

2) if type not field of collection user, query be:

query = user.find() 

i tried $exists not working me. , how can use $exists before querying? because collection, field name , field value dynamic. , how can check type field exists or not in user collection?

i tried solution $exists, want know if there way it.

as @philipp mentioned in comment, bound bump performance issues need perform full-collection scan without use of indexes. using $exists, query return fields equal null. suppose want first find records have field type set , isn't null, use combination of $ne operator. closest try count of field , determine query object based on field count:

var type_count = db.user.find({"type": {$exists: true, $ne: null}}).count(); var query = (type_count > 0) ? {"type": "article"} : {}; db.user.find(query);