fix(Databricks): round-trip display_name on measures (#326)

converters/databricks/README.md

@@ -86,10 +86,10 @@ Each row maps in both directions; the **Notes** flag where a behavior is specifi
 | `dataset.fields[]` | `dimensions[]` | Export: fields flatten into one list and a joined column is qualified by its full join path (`customer.c_name`; `customer.region.r_name` when nested). |
 | `field.expression.dialects[]` | `expr` | Export: prefer the `DATABRICKS` dialect, else `ANSI_SQL`. |
 | `metrics[]` | `measures[]` | Export: fact columns are referenced bare (`SUM(amount)`). |
-| `field.label` | `display_name` | |
+| `field.label` | dimension `display_name` | A measure's `display_name` has no `label` on the Apache Ossie metric shape, so it rides in the stash instead (see the `custom_extensions` row). |
 | `field` / `metric` `description` | `comment` | |
 | `ai_context.synonyms` | `synonyms` | |
-| `custom_extensions[DATABRICKS]` | `filter`, `window`, `format`, `rely`, `materialization` | Import stashes Metric View only features here; export restores them -- keeping `MV -> Apache Ossie -> MV` lossless. |
+| `custom_extensions[DATABRICKS]` | `filter`, `window`, `format`, `rely`, `materialization`, measure `display_name` | Import stashes Metric View only features here; export restores them -- keeping `MV -> Apache Ossie -> MV` lossless. |
 
 ## Requirements
 

converters/databricks/src/ossie_databricks/metric_view_to_ossie.py

@@ -53,6 +53,9 @@
 _MODEL_STASH_KEYS = ("filter", "parameters", "materialization")
 _JOIN_STASH_KEYS = ("rely", "cardinality")
 _COLUMN_STASH_KEYS = ("format", "window")
+# A dimension's display_name maps to the field `label`, but a metric has no `label`, so a
+# measure's display_name is stashed instead.
+_MEASURE_STASH_KEYS = ("display_name",)
 
 
 def _warn(scope, msg):
@@ -362,5 +365,6 @@ def _convert_measure(measure, fact_name):
         metric["description"] = measure["comment"]
     if measure.get("synonyms"):
         metric["ai_context"] = {"synonyms": list(measure["synonyms"])}
-    write_stash(metric, {k: measure[k] for k in _COLUMN_STASH_KEYS if k in measure})
+    stash_keys = _COLUMN_STASH_KEYS + _MEASURE_STASH_KEYS
+    write_stash(metric, {k: measure[k] for k in stash_keys if k in measure})
     return metric

converters/databricks/src/ossie_databricks/ossie_to_metric_view.py

@@ -571,6 +571,8 @@ def _convert_metric(metric, fact, seen_names):
         measure["format"] = stash["format"]
     if "window" in stash:
         measure["window"] = stash["window"]
+    if "display_name" in stash:
+        measure["display_name"] = stash["display_name"]
     return measure
 
 

converters/databricks/tests/_roundtrip_helpers.py

@@ -179,6 +179,8 @@ def build_metric_view(rnd):
         m = {"name": names.next("c"), "expr": f"{rnd.pick(_AGGS)}({rnd.colname()})"}
         if rnd.chance(0.4):
             m["comment"] = rnd.text()
+        if rnd.chance(0.3):
+            m["display_name"] = rnd.text()
         if rnd.chance(0.3):
             m["synonyms"] = [rnd.text() for _ in range(rnd.count(1, 3))]
         if rnd.chance(0.3):

converters/databricks/tests/test_roundtrip.py

@@ -79,6 +79,24 @@ def test_one_to_many_round_trips_mv_ossie_mv():
     assert parse(mv_out) == parse(mv_in)
 
 
+def test_measure_display_name_survives_round_trip_via_stash():
+    """A measure's display_name survives MV -> Apache Ossie -> MV. A dimension's
+    display_name maps to the Apache Ossie field `label`, but the metric shape has no
+    `label`, so a measure's display_name rides in the DATABRICKS stash (like format/window)
+    and is restored on the way back (apache/ossie#326)."""
+    mv_in = (
+        "version: '1.1'\nsource: c.s.fact\n"
+        "dimensions:\n- {name: region, expr: region}\n"
+        "measures:\n- {name: total_revenue, expr: SUM(amount), display_name: Total Revenue}\n"
+    )
+    ossie = importer.convert_metric_view_to_ossie(mv_in)
+    # The only display_name in the input is on the measure; it must ride in the metric's
+    # stash, since there is no native Apache Ossie field for it.
+    assert "display_name" in ossie
+    mv_out = exporter.convert_ossie_to_metric_view(ossie)
+    assert parse(mv_out) == parse(mv_in)
+
+
 # Property-based round-trip coverage. The Hypothesis driver lives in
 # test_roundtrip_properties.py; these run the same generators/assertions under a plain
 # seeded RNG so the property coverage also holds where Hypothesis is not installed.