id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
366510937,MDU6SXNzdWUzNjY1MTA5Mzc=,2460,Update docs to include how to Join using a non-index coord,5635139,open,0,5635139,,3,2018-10-03T20:19:15Z,2018-11-01T15:37:44Z,,MEMBER,,,,"I originally posted this on [SO](https://stackoverflow.com/questions/52625974/join-using-a-non-index-coord-in-xarray), as I thought it was a user question rather than a library issue. But after working on it more today, I'm not so sure.

I'm trying to do a 'join' in xarray, but using a non-index coordinate rather than a shared dim.

I have a Dataset indexed on 'a' with a coord on 'b', and a DataArray indexed on 'b':

```python
In [17]: ds=xr.Dataset(dict(a=(('x'),np.random.rand(10))), coords=dict(b=(('x'),list(range(10)))))

In [18]: ds
Out[18]:
<xarray.Dataset>
Dimensions:  (x: 10)
Coordinates:
    b        (x) int64 0 1 2 3 4 5 6 7 8 9
Dimensions without coordinates: x
Data variables:
    a        (x) float64 0.3634 0.2132 0.6945 0.5359 0.1053 0.07045 0.5945 ...

In [19]: da=xr.DataArray(np.random.rand(10), dims=('b',), coords=dict(b=(('b'),list(range(10)))))

In [20]: da
Out[20]:
<xarray.DataArray (b: 10)>
array([0.796987, 0.275992, 0.747882, 0.240374, 0.435143, 0.285271, 0.753582,
       0.556038, 0.365889, 0.434844])
Coordinates:
  * b        (b) int64 0 1 2 3 4 5 6 7 8 9

```

Can I add da onto my dataset, by joining on ds.b equalling da.b? The result would be:

```python
<xarray.Dataset>
Dimensions:  (x: 10)
Coordinates:
    b        (x) int64 0 1 2 3 4 5 6 7 8 9
Dimensions without coordinates: x
Data variables:
    a        (x) float64 0.3634 0.2132 0.6945 0.5359 0.1053 0.07045 0.5945 ...
    da       (x) float64 0.796987, 0.275992, 0.747882, 0.240374, 0.435143 ...
```

(for completeness - the data isn't current in the correct position)","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/2460/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue