fix: Use proper column name instead of attname

[last-partizan]

From django comment in db/models/fields/__init__.py:

# A guide to Field parameters:
#
#   * name:      The name of the field specified in the model.
#   * attname:   The attribute to use on the model object. This is the same as
#                "name", except in the case of ForeignKeys, where "_id" is
#                appended.
#   * db_column: The db_column specified in the model (or None).
#   * column:    The database column for this field. This is the same as
#                "attname", except if db_column is specified.
#
# Code that introspects values, or does other dynamic things, should use
# attname. For example, this gets the primary key value of object "obj":
#
#     getattr(obj, opts.pk.attname)

Problem

Currently there is not problem, unless someone used db_column on their parent link field.

Solution

This changes attribute name following django recommendations.

[foarsitter]

Do you think you can provide a test-case for the scenario you are describing?

[last-partizan]

Sure, updated test cases.

[foarsitter]

Thanks for providing a test case. I confirmed the issue and fix locally.

Can you update CHANGES.rst?

[codecov]

Codecov Report

Merging #573 (695b242) into master (6916342) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #573   +/-   ##
=======================================
  Coverage   95.12%   95.12%           
=======================================
  Files           6        6           
  Lines         820      820           
=======================================
  Hits          780      780           
  Misses         40       40           

Impacted Files	Coverage Δ
model_utils/managers.py	98.08% <ø> (ø)

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[foarsitter]

Thanks for your time and contribution @last-partizan!