Support nested models + related fixes

[dmach]

No description provided.

[pep8speaks]

Hello @dmach! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file osc/util/models.py:

Line 136:57: E721 do not compare types, use 'isinstance()'
Line 147:54: E721 do not compare types, use 'isinstance()'
Line 354:121: E501 line too long (139 > 120 characters)
Line 359:121: E501 line too long (152 > 120 characters)

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (472a56f) 31.24% compared to head (7903ade) 31.38%.
Report is 3 commits behind head on master.

Files	Patch %	Lines
osc/util/models.py	97.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1473      +/-   ##
==========================================
+ Coverage   31.24%   31.38%   +0.13%     
==========================================
  Files          49       49              
  Lines       17826    17859      +33     
==========================================
+ Hits         5570     5605      +35     
+ Misses      12256    12254       -2     

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