minor tweak to M2M so it flattens all_columns

[dantownsend]

Resolves #728

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.23%. Comparing base (d5123e9) to head (d5e8510).
⚠️ Report is 322 commits behind head on master.
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #730   +/-   ##
=======================================
  Coverage   91.22%   91.23%           
=======================================
  Files         106      107    +1     
  Lines        7292     7300    +8     
=======================================
+ Hits         6652     6660    +8     
  Misses        640      640           

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