def get_lemma_by_id(self, mongo_id):
cache_hit = None<EOL>if self._lemma_cache is not None:<EOL><INDENT>cache_hit = self._lemma_cache.get(mongo_id)<EOL><DEDENT>if cache_hit is not None:<EOL><INDENT>return cache_hit<EOL><DEDENT>lemma_dict = self._mongo_db.lexunits.find_one({'<STR_LIT>': mongo_id})<EOL>if lemma_dict is not None:<EOL><INDENT>lemma = Lemma(self, lemma_dict)<EOL>if self._lemma_cache is not None:<EOL><INDENT>self._lemma_cache.put(mongo_id, lemma)<EOL><DEDENT>return lemma<EOL><DEDENT>
Builds a Lemma object from the database entry with the given
ObjectId.

Arguments:
- `mongo_id`: a bson.objectid.ObjectId object