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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def find(key):
 """Return the value associated with a key. If there is no value with the given key, returns ``None``. """

    docs = list(collection.find({KEY_FIELD: key}))
    # Return None if we didn't find anything.
    if not docs:
        return None
    pickled_value = docs[0][VALUE_FIELD]
    # Unpickle and return the value.
    return pickle.loads(pickled_value)