Add a way to mark which dimensions populate others.

In the future, this will be used to make it easier to export dimension
data without knowing the details of the dimension combinations that
represent many-to-main joins (DM-34838).

There's no real gain to updating the dimensions configuration before
that happens, but merging these changes early is useful because it
will make software versions with just this change much better able to
handle future data repositories that use them in their dimensions
configuration, even if the associated functionality isn't available as
a result.

python/lsst/daf/butler/core/dimensions/_config.py

@@ -214,6 +214,7 @@ def _extractElementVisitors(self) -> Iterator[DimensionConstructionVisitor]:
                     metadata=metadata,
                     alwaysJoin=subconfig.get("always_join", False),
                     uniqueKeys=uniqueKeys,
+                    populated_by=subconfig.get("populated_by", None),
                 )
 
     def _extractTopologyVisitors(self) -> Iterator[DimensionConstructionVisitor]:

python/lsst/daf/butler/core/dimensions/_database.py

@@ -388,10 +388,12 @@ def __init__(
         implied: NamedValueAbstractSet[Dimension],
         metadata: NamedValueAbstractSet[ddl.FieldSpec],
         alwaysJoin: bool,
+        populated_by: Optional[Dimension],
     ):
         super().__init__(name, storage=storage, implied=implied, metadata=metadata)
         self._required = required
         self._alwaysJoin = alwaysJoin
+        self._populated_by = populated_by
 
     @property
     def required(self) -> NamedValueAbstractSet[Dimension]:
@@ -403,6 +405,11 @@ def alwaysJoin(self) -> bool:
         # Docstring inherited from DimensionElement.
         return self._alwaysJoin
 
+    @property
+    def populated_by(self) -> Optional[Dimension]:
+        # Docstring inherited.
+        return self._populated_by
+
 
 class DatabaseDimensionElementConstructionVisitor(DimensionConstructionVisitor):
     """Construction visitor for database dimension and dimension combination.
@@ -439,6 +446,9 @@ class DatabaseDimensionElementConstructionVisitor(DimensionConstructionVisitor):
         relationship between those dimensions that must always be satisfied.
         Should only be provided when a `DimensionCombination` is being
         constructed.
+    populated_by: `Dimension`, optional
+        The dimension that this element's records are always inserted,
+        exported, and imported alongside.
     """
 
     def __init__(
@@ -450,6 +460,7 @@ def __init__(
         metadata: Iterable[ddl.FieldSpec] = (),
         uniqueKeys: Iterable[ddl.FieldSpec] = (),
         alwaysJoin: bool = False,
+        populated_by: Optional[str] = None,
     ):
         super().__init__(name)
         self._storage = storage
@@ -458,6 +469,7 @@ def __init__(
         self._metadata = NamedValueSet(metadata).freeze()
         self._uniqueKeys = NamedValueSet(uniqueKeys).freeze()
         self._alwaysJoin = alwaysJoin
+        self._populated_by = populated_by
 
     def hasDependenciesIn(self, others: Set[str]) -> bool:
         # Docstring inherited from DimensionConstructionVisitor.
@@ -501,5 +513,8 @@ def visit(self, builder: DimensionConstructionBuilder) -> None:
                 implied=implied.freeze(),
                 metadata=self._metadata,
                 alwaysJoin=self._alwaysJoin,
+                populated_by=(
+                    builder.dimensions[self._populated_by] if self._populated_by is not None else None
+                ),
             )
             builder.elements.add(combination)

python/lsst/daf/butler/core/dimensions/_elements.py

@@ -330,6 +330,22 @@ def alwaysJoin(self) -> bool:
         """
         return False
 
+    @property
+    @abstractmethod
+    def populated_by(self) -> Optional[Dimension]:
+        """The dimension that this element's records are always inserted,
+        exported, and imported alongside.
+
+        Notes
+        -----
+        When this is `None` (as it will be, at least at first, for any data
+        repositories created before this attribute was added), records for
+        this element will often need to be exported manually when datasets
+        associated with some other related dimension are exported, in order for
+        the post-import data repository to function as expected.
+        """
+        raise NotImplementedError()
+
 
 class Dimension(DimensionElement):
     """A dimension.
@@ -376,6 +392,11 @@ def alternateKeys(self) -> NamedValueAbstractSet[ddl.FieldSpec]:
         _, *alternateKeys = self.uniqueKeys
         return NamedValueSet(alternateKeys).freeze()
 
+    @property
+    def populated_by(self) -> Dimension:
+        # Docstring inherited.
+        return self
+
 
 class DimensionCombination(DimensionElement):
     """Element with extra information.

python/lsst/daf/butler/core/dimensions/_universe.py

@@ -26,6 +26,7 @@
 import logging
 import math
 import pickle
+from collections import defaultdict
 from collections.abc import Iterable, Mapping
 from typing import TYPE_CHECKING, Any, ClassVar, TypeVar
 
@@ -170,6 +171,11 @@ def __new__(
         # and implied.  This is used for encode/decode.
         self._dimensionIndices = {name: i for i, name in enumerate(self._dimensions.names)}
 
+        self._populates = defaultdict(NamedValueSet)
+        for element in self._elements:
+            if element.populated_by is not None:
+                self._populates[element.populated_by.name].add(element)
+
         return self
 
     @property
@@ -483,6 +489,12 @@ def getEncodeLength(self) -> int:
         """
         return math.ceil(len(self._dimensions) / 8)
 
+    def get_elements_populated_by(self, dimension: Dimension) -> NamedValueAbstractSet[DimensionElement]:
+        """Return the set of `DimensionElement` objects whose
+        `~DimensionElement.populated_by` atttribute is the given dimension.
+        """
+        return self._populates[dimension.name]
+
     @classmethod
     def _unpickle(cls, version: int, namespace: str | None = None) -> DimensionUniverse:
         """Return an unpickled dimension universe.
@@ -542,6 +554,8 @@ def __deepcopy__(self, memo: dict) -> DimensionUniverse:
 
     _packers: dict[str, DimensionPackerFactory]
 
+    _populates: defaultdict[str, NamedValueSet]
+
     _version: int
 
     _namespace: str