expose memory key name (#808)

Author: hwchase17

langchain/chains/conversation/memory.py

@@ -230,20 +230,20 @@ class ConversationEntityMemory(Memory, BaseModel):
     llm: BaseLLM
     entity_extraction_prompt: BasePromptTemplate = ENTITY_EXTRACTION_PROMPT
     entity_summarization_prompt: BasePromptTemplate = ENTITY_SUMMARIZATION_PROMPT
-    memory_keys: List[str] = ["entities", "history"]  #: :meta private:
     output_key: Optional[str] = None
     input_key: Optional[str] = None
     store: Dict[str, Optional[str]] = {}
     entity_cache: List[str] = []
     k: int = 3
+    chat_history_key: str = "history"
 
     @property
     def memory_variables(self) -> List[str]:
         """Will always return list of memory variables.
 
         :meta private:
         """
-        return ["entities", "history"]
+        return ["entities", self.chat_history_key]
 
     def load_memory_variables(self, inputs: Dict[str, Any]) -> Dict[str, Any]:
         """Return history buffer."""
@@ -265,7 +265,7 @@ def load_memory_variables(self, inputs: Dict[str, Any]) -> Dict[str, Any]:
             entity_summaries[entity] = self.store.get(entity, "")
         self.entity_cache = entities
         return {
-            "history": "\n".join(self.buffer[-self.k :]),
+            self.chat_history_key: "\n".join(self.buffer[-self.k :]),
             "entities": entity_summaries,
         }