MongoDB return True if document exists

Note: This answer is outdated. More recent versions of MongoDB can use the far more efficient method db.collection.countDocuments. See the answer by Xavier Guihot for a better solution.

find doesn't return a boolean value, it returns a cursor. To check if that cursor contains any documents, use the cursor's count method:

if db.mycollection.find({'UserIDS': { "$in": newID}}).count() > 0

If newID is not an array you should not use the $in operator. You can simply do find({'UserIDS': newID}).

Starting Mongo 4.0.3/PyMongo 3.7.0, we can use count_documents:

if db.collection.count_documents({ 'UserIDS': newID }, limit = 1) != 0:
  # do something

Used with the optional parameter limit, this provides a way to find if there is at least one matching occurrence.

Limiting the number of matching occurrences makes the collection scan stop as soon as a match is found instead of going through the whole collection.

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Note that this can also be written as follow since 1 is interpreted as True in a python condition:

if db.collection.count_documents({ 'UserIDS': newID }, limit = 1):
  # do something

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In earlier versions of Mongo/Pymongo, count could be used (deprecated and replaced by count_documents in Mongo 4):

if db.collection.count({ 'UserIDS': newID }, limit = 1) != 0:
  # do something