Add method to get refcat schema from loader.

python/lsst/meas/algorithms/loadReferenceObjects.py

@@ -188,7 +188,7 @@ def __init__(self, dataIds, refCats, name=None, log=None, config=None):
             config = self.ConfigClass()
         self.config = config
         self.dataIds = dataIds
-        self.refCats = refCats
+        self.refCats = list(refCats)
         self.name = name
         self.log = log or logging.getLogger(__name__).getChild("ReferenceObjectLoader")
 
@@ -765,6 +765,47 @@ def loadSkyCircle(self, ctrCoord, radius, filterName, epoch=None):
         circularRegion = sphgeom.Circle(centerVector, sphRadius)
         return self.loadRegion(circularRegion, filterName, epoch=epoch)
 
+    def loadSchema(self, filterName):
+        """Load the schema for the reference catalog.
+
+        Parameters
+        ----------
+        filterName : `str`
+            Name of camera filter.
+
+         Returns
+        -------
+        output : `lsst.pipe.base.Struct`
+            Results struct with attributes:
+
+            ``schema``
+                Schema of the reference catalogs returned by other 'load'
+                methods.
+            ``fluxField``
+                Name of the field containing the flux associated with
+                ``filterName``.
+        """
+        if not self.refCats:
+            raise RuntimeError("No reference tables could be found.")
+        cat = self.refCats[0].get()
+        # Replace the original handle with an in-memory one that caches what
+        # we've already read, since there's a good chance we'll want to read it
+        # later.
+        self.refCats[0] = pipeBase.InMemoryDatasetHandle(cat, dataId=self.refCats[0].dataId, copy=False)
+        emptyCat = type(cat)(cat.table.clone())
+        expandedEmptyCat = self._remapReferenceCatalogSchema(
+            emptyCat,
+            anyFilterMapsToThis=self.config.anyFilterMapsToThis,
+            filterMap=self.config.filterMap,
+        )
+        fluxField = getRefFluxField(expandedEmptyCat.schema, filterName)
+        if expandedEmptyCat.schema[fluxField].asField().getUnits() != "nJy":
+            # if the flux field is not in nJy, check the refcat format version
+            version = getFormatVersionFromRefCat(emptyCat)
+            if version > LATEST_FORMAT_VERSION:
+                raise ValueError(f"Unsupported refcat format version: {version} > {LATEST_FORMAT_VERSION}.")
+        return pipeBase.Struct(schema=expandedEmptyCat.schema, fluxField=fluxField)
+
 
 def getRefFluxField(schema, filterName):
     """Get the name of a flux field from a schema.

tests/test_referenceObjectLoader.py

@@ -73,7 +73,7 @@ def testFilterAliasMap(self):
         for filterMap in ({}, {"camr": "r"}):
             config = ReferenceObjectLoader.ConfigClass()
             config.filterMap = filterMap
-            loader = ReferenceObjectLoader(None, None, name=None, config=config)
+            loader = ReferenceObjectLoader(None, [], name=None, config=config)
             refSchema = _makeSchema(filterNameList="r")
             loader._addFluxAliases(refSchema,
                                    anyFilterMapsToThis=config.anyFilterMapsToThis,
@@ -113,7 +113,7 @@ def testAnyFilterMapsToThisAlias(self):
         # test anyFilterMapsToThis
         config = ReferenceObjectLoader.ConfigClass()
         config.anyFilterMapsToThis = "gg"
-        loader = ReferenceObjectLoader(None, None, name=None, config=config)
+        loader = ReferenceObjectLoader(None, [], name=None, config=config)
         refSchema = _makeSchema(filterNameList=["gg"])
         loader._addFluxAliases(refSchema,
                                anyFilterMapsToThis=config.anyFilterMapsToThis,
@@ -129,7 +129,7 @@ def testAnyFilterMapsToThisAlias(self):
     def testGetMetadataCircle(self):
         center = lsst.geom.SpherePoint(100*lsst.geom.degrees, 45*lsst.geom.degrees)
         radius = lsst.geom.Angle(1*lsst.geom.degrees)
-        loader = ReferenceObjectLoader(None, None, name=None)
+        loader = ReferenceObjectLoader(None, [], name=None)
         metadata = loader.getMetadataCircle(center, radius, "fakeR")
         self.assertEqual(metadata['RA'], center.getLongitude().asDegrees())
         self.assertEqual(metadata['DEC'], center.getLatitude().asDegrees())