QueryBuilder: add flat keyword to first method

This keyword already exists for the `all` method and it will likewise be
useful for `first` when only a single quantity is projected. In that
case, often the caller doesn't want a list as a return value but simply
the projected quantity. Allowing to get this directly from the method
call as opposed to manually dereferencing the first item from the
returned list often makes for cleaner code.

aiida/orm/querybuilder.py

@@ -989,20 +989,28 @@ def _get_aiida_entity_res(value) -> Any:
         except TypeError:
             return value
 
-    def first(self) -> Optional[List[Any]]:
-        """Executes the query, asking for the first row of results.
+    def first(self, flat: bool = False) -> Optional[Union[List[Any], Any]]:
+        """Return the first result of the query.
 
-        Note, this may change if several rows are valid for the query,
-        as persistent ordering is not guaranteed unless explicitly specified.
+        Calling ``first`` results in an execution of the underlying query.
 
+        Note, this may change if several rows are valid for the query, as persistent ordering is not guaranteed unless
+        explicitly specified.
+
+        :param flat: if True, return just the projected quantity if there is just a single projection.
         :returns: One row of results as a list, or None if no result returned.
         """
         result = self._impl.first(self.as_dict())
 
         if result is None:
             return None
 
-        return [self._get_aiida_entity_res(rowitem) for rowitem in result]
+        result = [self._get_aiida_entity_res(rowitem) for rowitem in result]
+
+        if flat and len(result) == 1:
+            return result[0]
+
+        return result
 
     def count(self) -> int:
         """

tests/orm/test_querybuilder.py

@@ -649,7 +649,7 @@ def test_direction_keyword(self):
         assert res2 == {d2.id, d4.id}
 
     @staticmethod
-    def test_flat():
+    def test_all_flat():
         """Test the `flat` keyword for the `QueryBuilder.all()` method."""
         pks = []
         uuids = []
@@ -665,13 +665,26 @@ def test_flat():
         assert len(result) == 10
         assert result == pks
 
-        # Mutltiple projections
+        # Multiple projections
         builder = orm.QueryBuilder().append(orm.Data, project=['id', 'uuid']).order_by({orm.Data: 'id'})
         result = builder.all(flat=True)
         assert isinstance(result, list)
         assert len(result) == 20
         assert result == list(chain.from_iterable(zip(pks, uuids)))
 
+    @staticmethod
+    def test_first_flat():
+        """Test the `flat` keyword for the `QueryBuilder.first()` method."""
+        node = orm.Data().store()
+
+        # Single projected property
+        query = orm.QueryBuilder().append(orm.Data, project='id', filters={'id': node.pk})
+        assert query.first(flat=True) == node.pk
+
+        # Mutltiple projections
+        query = orm.QueryBuilder().append(orm.Data, project=['id', 'uuid'], filters={'id': node.pk})
+        assert query.first(flat=True) == [node.pk, node.uuid]
+
     def test_query_links(self):
         """Test querying for links"""
         d1, d2, d3, d4 = [orm.Data().store() for _ in range(4)]