diff --git a/redash/query_runner/cass.py b/redash/query_runner/cass.py
index 1a56b99374..fb5ce3ee5d 100644
--- a/redash/query_runner/cass.py
+++ b/redash/query_runner/cass.py
@@ -26,12 +26,6 @@ def generate_ssl_options_dict(protocol, cert_path=None):
     return ssl_options
 
 
-def custom_json_encoder(dec, o):
-    if isinstance(o, sortedset):
-        return list(o)
-    return None
-
-
 class Cassandra(BaseQueryRunner):
     noop_query = "SELECT dateof(now()) FROM system.local"
 
@@ -39,6 +33,12 @@ class Cassandra(BaseQueryRunner):
     def enabled(cls):
         return enabled
 
+    @classmethod
+    def custom_json_encoder(cls, dec, o):
+        if isinstance(o, sortedset):
+            return list(o)
+        return None
+
     @classmethod
     def configuration_schema(cls):
         return {
diff --git a/redash/query_runner/mongodb.py b/redash/query_runner/mongodb.py
index e22d76ca00..b775edb4f8 100644
--- a/redash/query_runner/mongodb.py
+++ b/redash/query_runner/mongodb.py
@@ -42,16 +42,6 @@
 }
 
 
-def custom_json_encoder(dec, o):
-    if isinstance(o, ObjectId):
-        return str(o)
-    elif isinstance(o, Timestamp):
-        return dec.default(o.as_datetime())
-    elif isinstance(o, Decimal128):
-        return o.to_decimal()
-    return None
-
-
 date_regex = re.compile(r'ISODate\("(.*)"\)', re.IGNORECASE)
 
 
@@ -170,6 +160,16 @@ def __init__(self, configuration):
             True if "replicaSetName" in self.configuration and self.configuration["replicaSetName"] else False
         )
 
+    @classmethod
+    def custom_json_encoder(cls, dec, o):
+        if isinstance(o, ObjectId):
+            return str(o)
+        elif isinstance(o, Timestamp):
+            return dec.default(o.as_datetime())
+        elif isinstance(o, Decimal128):
+            return o.to_decimal()
+        return None
+
     def _get_db(self):
         kwargs = {}
         if self.is_replica_set:
diff --git a/redash/query_runner/pg.py b/redash/query_runner/pg.py
index 6b9422a74c..dc74aff2cc 100644
--- a/redash/query_runner/pg.py
+++ b/redash/query_runner/pg.py
@@ -55,18 +55,6 @@
 }
 
 
-def custom_json_encoder(dec, o):
-    if isinstance(o, Range):
-        # From: https://github.com/psycopg/psycopg2/pull/779
-        if o._bounds is None:
-            return ""
-
-        items = [o._bounds[0], str(o._lower), ", ", str(o._upper), o._bounds[1]]
-
-        return "".join(items)
-    return None
-
-
 def _wait(conn, timeout=None):
     while 1:
         try:
@@ -195,6 +183,18 @@ def configuration_schema(cls):
     def type(cls):
         return "pg"
 
+    @classmethod
+    def custom_json_encoder(cls, dec, o):
+        if isinstance(o, Range):
+            # From: https://github.com/psycopg/psycopg2/pull/779
+            if o._bounds is None:
+                return ""
+
+            items = [o._bounds[0], str(o._lower), ", ", str(o._upper), o._bounds[1]]
+
+            return "".join(items)
+        return None
+
     def _get_definitions(self, schema, query):
         results, error = self.run_query(query, None)
 
diff --git a/redash/utils/__init__.py b/redash/utils/__init__.py
index f03767606c..a4005b6725 100644
--- a/redash/utils/__init__.py
+++ b/redash/utils/__init__.py
@@ -74,7 +74,9 @@ class JSONEncoder(json.JSONEncoder):
     """Adapter for `json.dumps`."""
 
     def __init__(self, **kwargs):
-        self.encoders = [m.custom_json_encoder for m in sys.modules.values() if hasattr(m, "custom_json_encoder")]
+        from redash.query_runner import query_runners
+
+        self.encoders = [r.custom_json_encoder for r in query_runners.values() if hasattr(r, "custom_json_encoder")]
         super().__init__(**kwargs)
 
     def default(self, o):