diff --git a/datagateway_api/common/database/filters.py b/datagateway_api/common/database/filters.py
index 6bb41031..ffce968f 100644
--- a/datagateway_api/common/database/filters.py
+++ b/datagateway_api/common/database/filters.py
@@ -72,27 +72,30 @@ def _add_query_join(self, query):
         :return: Entity model of the field (usually the field relating to the endpoint
             the request is coming from)
         """
-        try:
-            field = getattr(query.table, self.field)
-        except AttributeError:
-            raise FilterError(
-                f"Unknown attribute {self.field} on table {query.table.__name__}",
-            )
 
         if self.related_related_field:
-            included_table = getattr(models, self.field)
-            included_included_table = getattr(models, self.related_field)
+            included_table = get_entity_object_from_name(self.field)
+            included_included_table = get_entity_object_from_name(self.related_field)
             query.base_query = query.base_query.join(included_table).join(
                 included_included_table,
             )
-            field = getattr(included_included_table, self.related_related_field)
+            field = self._get_field(included_included_table, self.related_related_field)
         elif self.related_field:
             included_table = get_entity_object_from_name(self.field)
             query.base_query = query.base_query.join(included_table)
-            field = getattr(included_table, self.related_field)
+            field = self._get_field(included_table, self.related_field)
+        else:
+            # No related fields
+            field = self._get_field(query.table, self.field)
 
         return field
 
+    def _get_field(self, table, field):
+        try:
+            return getattr(table, field)
+        except AttributeError:
+            raise FilterError(f"Unknown attribute {field} on table {table.__name__}")
+
 
 class DatabaseWhereFilter(WhereFilter, DatabaseFilterUtilities):
     def __init__(self, field, value, operation):