diff --git a/xarray_ms/backend/msv2/entrypoint.py b/xarray_ms/backend/msv2/entrypoint.py
index 75107e2..6f44edb 100644
--- a/xarray_ms/backend/msv2/entrypoint.py
+++ b/xarray_ms/backend/msv2/entrypoint.py
@@ -77,9 +77,8 @@ def initialise_default_args(
   epoch: str | None,
   table_factory: TableFactory | None,
   partition_columns: List[str] | None,
-  partition_key: PartitionKeyT | None,
   structure_factory: MSv2StructureFactory | None,
-) -> Tuple[str, TableFactory, List[str], PartitionKeyT, MSv2StructureFactory]:
+) -> Tuple[str, TableFactory, List[str], MSv2StructureFactory]:
   """
   Ensures consistency when initialising default arguments from multiple locations
   """
@@ -98,16 +97,7 @@ def initialise_default_args(
   structure_factory = structure_factory or MSv2StructureFactory(
     table_factory, partition_columns, auto_corrs=auto_corrs
   )
-  structure = structure_factory()
-  if partition_key is None:
-    partition_key = next(iter(structure.keys()))
-    warnings.warn(
-      f"No partition_key was supplied. Selected first partition {partition_key}"
-    )
-  elif partition_key not in structure:
-    raise ValueError(f"{partition_key} not in {list(structure.keys())}")
-
-  return epoch, table_factory, partition_columns, partition_key, structure_factory
+  return epoch, table_factory, partition_columns, structure_factory
 
 
 class MSv2Store(AbstractWritableDataStore):
@@ -164,7 +154,7 @@ def open(
     if not isinstance(ms, str):
       raise ValueError("Measurement Sets paths must be strings")
 
-    epoch, table_factory, partition_columns, partition_key, structure_factory = (
+    epoch, table_factory, partition_columns, structure_factory = (
       initialise_default_args(
         ms,
         ninstances,
@@ -172,11 +162,20 @@ def open(
         epoch,
         None,
         partition_columns,
-        partition_key,
         structure_factory,
       )
     )
 
+    structure = structure_factory()
+
+    if partition_key is None:
+      partition_key = next(iter(structure.keys()))
+      warnings.warn(
+        f"No partition_key was supplied. Selected first partition {partition_key}"
+      )
+    elif partition_key not in structure:
+      raise ValueError(f"{partition_key} not in {list(structure.keys())}")
+
     return cls(
       table_factory,
       structure_factory,
@@ -332,8 +331,8 @@ def open_datatree(
     else:
       raise ValueError("Measurement Set paths must be strings")
 
-    epoch, _, partition_columns, _, structure_factory = initialise_default_args(
-      ms, ninstances, auto_corrs, epoch, None, partition_columns, None, None
+    epoch, _, partition_columns, structure_factory = initialise_default_args(
+      ms, ninstances, auto_corrs, epoch, None, partition_columns, None
     )
 
     structure = structure_factory()
@@ -341,7 +340,7 @@ def open_datatree(
     chunks = kwargs.pop("chunks", None)
     pchunks = promote_chunks(structure, chunks)
 
-    for i, partition_key in enumerate(structure):
+    for partition_key in structure:
       ds = xarray.open_dataset(
         ms,
         drop_variables=drop_variables,
@@ -354,6 +353,8 @@ def open_datatree(
         chunks=None if pchunks is None else pchunks[partition_key],
         **kwargs,
       )
-      datasets[str(i)] = ds
+
+      key = ",".join(f"{k}={v}" for k, v in sorted(partition_key))
+      datasets[key] = ds
 
     return DataTree.from_dict(datasets)