Rework failure message for RowMatchingEquality (#100)

* Rework failure message.

* Add changelog entry.

CHANGELOG.rst

@@ -17,6 +17,10 @@ Changelog
 - Added a ``name`` parameter to all ``add_x_constraint`` methods of ``WithinRequirement`` and ``BetweenRequirement``. This will give pytest test a custom name.
 - Added preliminary support for Impala.
 
+**Other changes**
+
+- Improve assertion error for :meth:`~datajudge.WithinRequirement.add_row_matching_equality_constraint`.
+
 
 1.2.0 - 2022.10.21
 ------------------

src/datajudge/constraints/row.py

@@ -177,7 +177,7 @@ def __init__(
         )
 
     def test(self, engine: sa.engine.Engine) -> TestResult:
-        missing_fraction, selections = db_access.get_row_mismatch(
+        missing_fraction, n_rows_match, selections = db_access.get_row_mismatch(
             engine, self.ref, self.ref2, self.match_and_compare
         )
         self.factual_selections = selections
@@ -187,10 +187,10 @@ def test(self, engine: sa.engine.Engine) -> TestResult:
             return TestResult.success()
         assertion_message = (
             f"{missing_fraction} > "
-            f"{max_missing_fraction} of rows matched "
-            f"between {self.ref.get_string()} and "
+            f"{max_missing_fraction} of the rows differ "
+            f"on a match of {n_rows_match} rows between {self.ref.get_string()} and "
             f"{self.ref2.get_string()}. "
-            f"{self.condition_string}."
+            f"{self.condition_string}"
             f"{self.match_and_compare} "
         )
         return TestResult.failure(assertion_message)

src/datajudge/db_access.py

@@ -924,11 +924,15 @@ def get_row_mismatch(engine, ref, ref2, match_and_compare):
 
     avg_match_column = sa.func.avg(sa.case([(compare, 0.0)], else_=1.0))
 
-    selection = sa.select([avg_match_column]).select_from(
+    selection_difference = sa.select([avg_match_column]).select_from(
         subselection1.join(subselection2, match)
     )
-    result = engine.connect().execute(selection).scalar()
-    return result, [selection]
+    selection_n_rows = sa.select(sa.func.count()).select_from(
+        subselection1.join(subselection2, match)
+    )
+    result_mismatch = engine.connect().execute(selection_difference).scalar()
+    result_n_rows = engine.connect().execute(selection_n_rows).scalar()
+    return result_mismatch, result_n_rows, [selection_difference, selection_n_rows]
 
 
 def get_duplicate_sample(engine, ref):

tests/integration/test_integration.py

@@ -1926,7 +1926,8 @@ def test_row_matching_equality(engine, row_match_table1, row_match_table2, data)
         condition1=condition1,
         condition2=condition2,
     )
-    assert operation(req[0].test(engine).outcome)
+    test_result = req[0].test(engine)
+    assert operation(test_result.outcome), test_result.failure_message
 
 
 @pytest.mark.parametrize("key", [("some_id",), ("some_id", "extra_id")])