Allow id to be used in file template

If id is used it must at least be present in the file
name part of the template path.

python/lsst/daf/butler/core/fileTemplates.py

@@ -338,7 +338,7 @@ class FileTemplate:
     template.
     """
 
-    mandatoryFields = {"run"}
+    mandatoryFields = {"run", "id"}
     """A set of fields, one of which must be present in a template."""
 
     datasetFields = {"datasetType", "component"}
@@ -462,6 +462,9 @@ def format(self, ref: DatasetRef) -> str:
         usedRun = False
         fields["run"] = ref.run
 
+        usedId = False
+        fields["id"] = ref.id
+
         fmt = string.Formatter()
         parts = fmt.parse(self.template)
         output = ""
@@ -486,8 +489,11 @@ def format(self, ref: DatasetRef) -> str:
             else:
                 optional = False
 
+            # We must use at least a run or id.
             if field_name == "run":
                 usedRun = True
+            if field_name == "id":
+                usedId = True
 
             if field_name == "collection":
                 raise KeyError(
@@ -552,9 +558,11 @@ def format(self, ref: DatasetRef) -> str:
                 "Component '{}' specified but template {} did not use it".format(component, self.template)
             )
 
-        # Complain if there's no run
-        if not usedRun:
-            raise KeyError("Template does not include 'run'.")
+        # Complain if there's no run or id
+        if not usedRun and not usedId:
+            missing = ("run" if not usedRun else None, "id" if not usedId else None)
+            text = " or ".join(f"'{m}'" for m in missing if m is not None)
+            raise KeyError(f"Template does not include {text}.")
 
         # Since this is known to be a path, normalize it in case some double
         # slashes have crept in
@@ -596,12 +604,24 @@ def validateTemplate(self, entity: Union[DatasetRef, DatasetType, StorageClass,
             )
 
         # Check that there are some dimension fields in the template
+        # The id is allowed instead if present since that also uniquely
+        # identifies the file in the datastore.
         allfields = self.fields(optionals=True)
-        if not allfields:
+        if not allfields and "id" not in withSpecials:
             raise FileTemplateValidationError(
                 f"Template '{self}' does not seem to have any fields corresponding to dimensions."
             )
 
+        # Require that if "id" is in the template then it must exist in the
+        # file part -- this avoids templates like "{id}/fixed" where the file
+        # name is fixed but the directory has the ID.
+        if "id" in withSpecials:
+            file_part = os.path.split(self.template)[-1]
+            if "{id}" not in file_part:
+                raise FileTemplateValidationError(
+                    f"Template '{self}' includes the 'id' but that ID is not part of the file name."
+                )
+
         # If we do not have dimensions available then all we can do is shrug
         if not hasattr(entity, "dimensions"):
             return

tests/config/basic/templates.yaml

@@ -7,6 +7,7 @@ test_metric_comp: "{run:/}/{datasetType}.{component:?}/{datasetType}_v{visit:08d
 metric2: "{run:/}/{datasetType}.{component:?}/{tract:?}/{patch:?}/{physical_filter:?}/{instrument:?}_{visit.name:?}"
 metric3: "{run:/}/{datasetType}/{instrument}"
 metric4: "{run:/}/{component:?}_{instrument}_{physical_filter}_{visit:08d}"
+Integer: "{id}"
 physical_filter+: "{run:/}/{instrument}_{physical_filter}"
 instrument<DummyCamComp>:
   metric33: "{run:/}/{instrument}/{datasetType}.{component:?}/{datasetType}_v{visit:08d}_f{physical_filter}_{component:?}"

tests/test_templates.py

@@ -98,6 +98,23 @@ def testBasic(self):
             self.makeDatasetRef("calexp", run="run/2", conform=False),
         )
 
+        # Check that the id is sufficient without any other information.
+        self.assertTemplate("{id}", "1", self.makeDatasetRef("calexp", run="run2", conform=False))
+
+        self.assertTemplate("{run}/{id}", "run2/1", self.makeDatasetRef("calexp", run="run2", conform=False))
+
+        self.assertTemplate(
+            "fixed/{id}",
+            "fixed/1",
+            self.makeDatasetRef("calexp", run="run2", conform=False),
+        )
+
+        self.assertTemplate(
+            "fixed/{id}_{physical_filter}",
+            "fixed/1_Most_Amazing_U_Filter_Ever",
+            self.makeDatasetRef("calexp", run="run2", conform=False),
+        )
+
         # Retain any "/" in run
         tmplstr = "{run:/}/{datasetType}/{visit:05d}/{physical_filter}-trail-{run}"
         self.assertTemplate(
@@ -123,6 +140,9 @@ def testBasic(self):
         with self.assertRaises(FileTemplateValidationError):
             FileTemplate("{run}_{datasetType}")
 
+        with self.assertRaises(FileTemplateValidationError):
+            FileTemplate("{id}/fixed")
+
     def testRunOrCollectionNeeded(self):
         tmplstr = "{datasetType}/{visit:05d}/{physical_filter}"
         with self.assertRaises(FileTemplateValidationError):
@@ -346,6 +366,12 @@ def testValidation(self):
             dataId={"physical_filter": "i", "instrument": "HSC"},
         )
 
+        entities["metric6"] = self.makeDatasetRef(
+            "filter_inst",
+            storageClassName="Integer",
+            dataId={"physical_filter": "i", "instrument": "HSC"},
+        )
+
         templates.validateTemplates(entities.values(), logFailures=True)
 
         # Rerun but with a failure