html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/pull/3312#issuecomment-532754800,https://api.github.com/repos/pydata/xarray/issues/3312,532754800,MDEyOklzc3VlQ29tbWVudDUzMjc1NDgwMA==,10554254,2019-09-18T16:08:09Z,2019-09-18T16:08:09Z,NONE,Opened https://github.com/pydata/xarray/issues/3315 regarding combine_nested() failing when being passed nested DataSets.,"{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,494210818
https://github.com/pydata/xarray/pull/3312#issuecomment-532419859,https://api.github.com/repos/pydata/xarray/issues/3312,532419859,MDEyOklzc3VlQ29tbWVudDUzMjQxOTg1OQ==,10554254,2019-09-17T22:03:23Z,2019-09-17T23:51:13Z,NONE,"`pytest -q xarray/tests/test_combine.py` is telling me that 

```
def test_concat_name_symmetry(self):
    """"""Inspired by the discussion on GH issue #2777""""""

    da1 = DataArray(name=""a"", data=[[0]], dims=[""x"", ""y""])
    da2 = DataArray(name=""b"", data=[[1]], dims=[""x"", ""y""])
    da3 = DataArray(name=""a"", data=[[2]], dims=[""x"", ""y""])
    da4 = DataArray(name=""b"", data=[[3]], dims=[""x"", ""y""])

    x_first = combine_nested([[da1, da2], [da3, da4]], concat_dim=[""x"", ""y""])
```

fails with:
```
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-13-bc4a941bd0c3> in <module>
      3 da3 = xr.DataArray(name=""a"", data=[[2]], dims=[""x"", ""y""])
      4 da4 = xr.DataArray(name=""b"", data=[[3]], dims=[""x"", ""y""])
----> 5 xr.combine_nested([[da1, da2], [da3, da4]], concat_dim=[""x"", ""y""])

~/repos/contribute/xarray/xarray/core/combine.py in combine_nested(objects, concat_dim, compat, data_vars, coords, fill_value, join)
    468         ids=False,
    469         fill_value=fill_value,
--> 470         join=join,
    471     )
    472 

~/repos/contribute/xarray/xarray/core/combine.py in _nested_combine(datasets, concat_dims, compat, data_vars, coords, ids, fill_value, join)
    305         coords=coords,
    306         fill_value=fill_value,
--> 307         join=join,
    308     )
    309     return combined

~/repos/contribute/xarray/xarray/core/combine.py in _combine_nd(combined_ids, concat_dims, data_vars, coords, compat, fill_value, join)
    196             compat=compat,
    197             fill_value=fill_value,
--> 198             join=join,
    199         )
    200     (combined_ds,) = combined_ids.values()

~/repos/contribute/xarray/xarray/core/combine.py in _combine_all_along_first_dim(combined_ids, dim, data_vars, coords, compat, fill_value, join)
    218         datasets = combined_ids.values()
    219         new_combined_ids[new_id] = _combine_1d(
--> 220             datasets, dim, compat, data_vars, coords, fill_value, join
    221         )
    222     return new_combined_ids

~/repos/contribute/xarray/xarray/core/combine.py in _combine_1d(datasets, concat_dim, compat, data_vars, coords, fill_value, join)
    246                 compat=compat,
    247                 fill_value=fill_value,
--> 248                 join=join,
    249             )
    250         except ValueError as err:

~/repos/contribute/xarray/xarray/core/concat.py in concat(objs, dim, data_vars, coords, compat, positions, fill_value, join)
    131             ""objects, got %s"" % type(first_obj)
    132         )
--> 133     return f(objs, dim, data_vars, coords, compat, positions, fill_value, join)
    134 
    135 

~/repos/contribute/xarray/xarray/core/concat.py in _dataset_concat(datasets, dim, data_vars, coords, compat, positions, fill_value, join)
    363     for k in datasets[0].variables:
    364         if k in concat_over:
--> 365             vars = ensure_common_dims([ds.variables[k] for ds in datasets])
    366             combined = concat_vars(vars, dim, positions)
    367             assert isinstance(combined, Variable)

~/repos/contribute/xarray/xarray/core/concat.py in <listcomp>(.0)
    363     for k in datasets[0].variables:
    364         if k in concat_over:
--> 365             vars = ensure_common_dims([ds.variables[k] for ds in datasets])
    366             combined = concat_vars(vars, dim, positions)
    367             assert isinstance(combined, Variable)

~/repos/contribute/xarray/xarray/core/utils.py in __getitem__(self, key)
    383 
    384     def __getitem__(self, key: K) -> V:
--> 385         return self.mapping[key]
    386 
    387     def __iter__(self) -> Iterator[K]:

KeyError: 'a'
```

It looks like the existing combine_nested() routine actually wants a DataArray and fails if passed a DataSet.

The following should work with current master.

```
da1 = xr.DataArray(name=""a"", data=[[0]], dims=[""x"", ""y""])
da2 = xr.DataArray(name=""b"", data=[[1]], dims=[""x"", ""y""])
da3 = xr.DataArray(name=""a"", data=[[2]], dims=[""x"", ""y""])
da4 = xr.DataArray(name=""b"", data=[[3]], dims=[""x"", ""y""])
xr.combine_nested([[da1, da2], [da3, da4]], concat_dim=[""x"", ""y""])
```

While converting to DataSet will cause the same error expressed by the test.

```
ds1 = da1.to_dataset()
ds2 = da2.to_dataset()
ds3 = da3.to_dataset()
ds4 = da4.to_dataset()
xr.combine_nested([[ds1, ds2], [ds3, ds4]], concat_dim=[""x"", ""y""])
```","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,494210818