MongoDB return True if document exists

Question

I want to return true if a userID already exists and false otherwise from my collection.I have this function but it always returns True.

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Answer 1

If you're using Motor, find() doesn't do any communication with the database, it merely creates and returns a MotorCursor:

http://motor.readthedocs.org/en/stable/api/motor_collection.html#motor.MotorCollection.find

Since the MotorCursor is not None, Python considers it a "true" value so your function returns True. If you want to know if at least one document exists that matches your query, try find_one():

@gen.coroutine
def alreadyExists(newID):
    doc = yield db.mycollection.find_one({'UserIDS': { "$in": newID}})
    return bool(doc)

Notice you need a "coroutine" and "yield" to do I/O with Tornado. You could also use a callback:

def alreadyExists(newID, callback):
    db.mycollection.find_one({'UserIDS': { "$in": newID}}, callback=callback)

For more on callbacks and coroutines, see the Motor tutorial:

http://motor.readthedocs.org/en/stable/tutorial.html

If you're using PyMongo and not Motor, it's simpler:

def alreadyExists(newID):
    return bool(db.mycollection.find_one({'UserIDS': { "$in": newID}}))

Final note, MongoDB's $in operator takes a list of values. Is newID a list? Perhaps you just want:

find_one({'UserIDS': newID})