Fix bug with deconstruction when using choices, choices_display

Fixes #74

README.md

@@ -97,6 +97,7 @@ poetry run pytest
 #### Unreleased
 
 * Officially support for django 3.2, python 3.9
+* Fix bug with field deconstruction ([#74](https://github.com/mfogel/django-timezone-field/issues/74))
 
 #### 4.1.2 (2021-03-17)
 

tests/test_deconstruct.py

@@ -1,3 +1,4 @@
+import pytest
 import pytz
 
 from django.db.migrations.writer import MigrationWriter
@@ -95,3 +96,59 @@ def test_specifying_defaults_not_frozen():
     field = TimeZoneField(choices=choices)
     name, path, args, kwargs = field.deconstruct()
     assert 'choices' not in kwargs
+
+
+@pytest.mark.parametrize('choices', [[
+    (pytz.timezone('US/Pacific'), 'US/Pacific'),
+    (pytz.timezone('US/Eastern'), 'US/Eastern'),
+], [
+    ('US/Pacific', 'US/Pacific'),
+    ('US/Eastern', 'US/Eastern'),
+]])
+def test_deconstruct_when_using_just_choices(choices):
+    field = TimeZoneField(choices=choices)
+    name, path, args, kwargs = field.deconstruct()
+    assert kwargs == {'choices': [
+        ('US/Pacific', 'US/Pacific'),
+        ('US/Eastern', 'US/Eastern'),
+    ]}
+
+
+@pytest.mark.parametrize('choices_display, expected_kwargs', [
+    [None, {}],
+    ['STANDARD', {'choices_display': 'STANDARD'}],
+    ['WITH_GMT_OFFSET', {'choices_display': 'WITH_GMT_OFFSET'}],
+])
+def test_deconstruct_when_using_just_choices_display(choices_display, expected_kwargs):
+    field = TimeZoneField(choices_display=choices_display)
+    name, path, args, kwargs = field.deconstruct()
+    assert kwargs == expected_kwargs
+
+
+@pytest.mark.parametrize('choices, choices_display, expected_kwargs', [
+    [
+        [['US/Pacific', 'West Coast Time']],
+        None,
+        {'choices': [('US/Pacific', 'West Coast Time')]},
+    ],
+    [
+        [['US/Pacific', 'West Coast Time']],
+        'STANDARD',
+        {'choices': [('US/Pacific', '')], 'choices_display': 'STANDARD'},
+    ],
+    [
+        [['US/Pacific', 'West Coast Time']],
+        'WITH_GMT_OFFSET',
+        {'choices': [('US/Pacific', '')], 'choices_display': 'WITH_GMT_OFFSET'},
+    ],
+    [
+        [[tz, 'ignored'] for tz in pytz.common_timezones],
+        'WITH_GMT_OFFSET',
+        {'choices_display': 'WITH_GMT_OFFSET'},
+    ],
+])
+def test_deconstruct_when_using_choices_and_choices_display(
+        choices, choices_display, expected_kwargs):
+    field = TimeZoneField(choices=choices, choices_display=choices_display)
+    name, path, args, kwargs = field.deconstruct()
+    assert kwargs == expected_kwargs

timezone_field/fields.py

@@ -37,7 +37,6 @@ class TimeZoneField(models.Field):
     # NOTE: these defaults are excluded from migrations. If these are changed,
     #       existing migration files will need to be accomodated.
     default_tzs = [pytz.timezone(tz) for tz in pytz.common_timezones]
-    default_choices = standard(default_tzs)
     default_max_length = 63
 
     def __init__(self, *args, **kwargs):
@@ -67,17 +66,17 @@ def __init__(self, *args, **kwargs):
             values = self.default_tzs
             displays = None
 
-        choices_display = kwargs.pop('choices_display', None)
-        if choices_display == 'WITH_GMT_OFFSET':
+        self.choices_display = kwargs.pop('choices_display', None)
+        if self.choices_display == 'WITH_GMT_OFFSET':
             choices = with_gmt_offset(values)
-        elif choices_display == 'STANDARD':
+        elif self.choices_display == 'STANDARD':
             choices = standard(values)
-        elif choices_display is None:
+        elif self.choices_display is None:
             choices = zip(values, displays) if displays else standard(values)
         else:
             raise ValueError(
                 "Unrecognized value for kwarg 'choices_display' of '"
-                + choices_display + "'"
+                + self.choices_display + "'"
             )
 
         # 'display_GMT_offset' is deprecated, use 'choices_display' instead
@@ -94,11 +93,23 @@ def validate(self, value, model_instance):
 
     def deconstruct(self):
         name, path, args, kwargs = super(TimeZoneField, self).deconstruct()
-        if kwargs.get('choices') == self.default_choices:
-            del kwargs['choices']
         if kwargs.get('max_length') == self.default_max_length:
             del kwargs['max_length']
 
+        if self.choices_display is not None:
+            kwargs['choices_display'] = self.choices_display
+
+        choices = kwargs['choices']
+        if self.choices_display is None:
+            if choices == standard(self.default_tzs):
+                kwargs.pop('choices')
+        else:
+            values, _ = zip(*choices)
+            if sorted(values, key=str) == sorted(self.default_tzs, key=str):
+                kwargs.pop('choices')
+            else:
+                kwargs['choices'] = [(value, '') for value in values]
+
         # django can't decontruct pytz objects, so transform choices
         # to [<str>, <str>] format for writing out to the migration
         if 'choices' in kwargs: