#141: Add manual_count flag

- Flag used for a count request that has a distinct filter

datagateway_api/common/icat/filters.py

@@ -119,13 +119,22 @@ def apply_filter(self, query):
             log.info("Adding ICAT distinct filter to ICAT query")
             log.debug("Fields for distinct filter: %s", self.fields)
 
-            if (
-                query.aggregate == "COUNT"
-                or query.aggregate == "AVG"
-                or query.aggregate == "SUM"
-            ):
+            # These aggregate keywords not currently used in the API, but conditional
+            # present in case they're used in the future
+            if query.aggregate == "AVG" or query.aggregate == "SUM":
                 # Distinct can be combined with other aggregate functions
                 query.setAggregate(f"{query.aggregate}:DISTINCT")
+            elif query.aggregate == "COUNT":
+                # When count and distinct keywords are used together when selecting
+                # multiple attributes, Python ICAT will always throw an error on query
+                # execution (more info:
+                # https://github.com/icatproject/python-icat/issues/76). This appears to
+                # be a JPQL limitation, something that cannot be fixed in Python ICAT.
+                # As a result, the API will get the distinct results and manually
+                # perform `len()` on the list, using `manual_count` as a flag to
+                # recognise this situation
+                query.setAggregate("DISTINCT")
+                query.manual_count = True
             else:
                 query.setAggregate("DISTINCT")
 

datagateway_api/common/icat/query.py

@@ -37,6 +37,8 @@ def __init__(
         :raises PythonICATError: If a ValueError is raised when creating a Query(), 500
             will be returned as a response
         """
+        # Flag for a count request that uses a distinct filter
+        self.manual_count = False
 
         try:
             log.info("Creating ICATQuery for entity: %s", entity_name)