950 ModelBuilder and recursive foreign keys

[dantownsend]

Resolves #950

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 90.31%. Comparing base (c55f8cf) to head (4ddcc66).

Files	Patch %	Lines
piccolo/testing/model_builder.py	83.33%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #951      +/-   ##
==========================================
- Coverage   92.01%   90.31%   -1.71%     
==========================================
  Files         108      108              
  Lines        8233     8237       +4     
==========================================
- Hits         7576     7439     -137     
- Misses        657      798     +141     

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