use custom JSON type for database for more generic support (#389)

ragna/deploy/_api/database.py

@@ -129,7 +129,7 @@ def _orm_to_schema_chat(chat: orm.Chat) -> schemas.Chat:
             documents=documents,
             source_storage=chat.source_storage,
             assistant=chat.assistant,
-            params=chat.params,  # type: ignore[arg-type]
+            params=chat.params,
         ),
         messages=messages,
         prepared=chat.prepared,

ragna/deploy/_api/orm.py

@@ -1,9 +1,35 @@
+import json
+from typing import Any
+
 from sqlalchemy import Column, ForeignKey, Table, types
+from sqlalchemy.engine import Dialect
 from sqlalchemy.orm import DeclarativeBase, relationship  # type: ignore[attr-defined]
 
 from ragna.core import MessageRole
 
 
+class Json(types.TypeDecorator):
+    """Universal JSON type which stores values as strings.
+
+    This is needed because sqlalchemy.types.JSON only works for a limited subset of
+    databases.
+    """
+
+    impl = types.String
+
+    cache_ok = True
+
+    def process_bind_param(self, value: Any, dialect: Dialect) -> str:
+        return json.dumps(value)
+
+    def process_result_value(
+        self,
+        value: str,  # type: ignore[override]
+        dialect: Dialect,
+    ) -> Any:
+        return json.loads(value)
+
+
 class Base(DeclarativeBase):
     pass
 
@@ -34,7 +60,7 @@ class Document(Base):
     name = Column(types.String)
     # Mind the trailing underscore here. Unfortunately, this is necessary, because
     # metadata without the underscore is reserved by SQLAlchemy
-    metadata_ = Column(types.JSON)
+    metadata_ = Column(Json)
     chats = relationship(
         "Chat",
         secondary=document_chat_association_table,
@@ -59,7 +85,7 @@ class Chat(Base):
     )
     source_storage = Column(types.String)
     assistant = Column(types.String)
-    params = Column(types.JSON)
+    params = Column(Json)
     messages = relationship("Message", cascade="all, delete")
     prepared = Column(types.Boolean)