Re: [Django] #34027: When ForeignKey target field type is changed from CharField to IntegerField, varchar_pattern_ops index is not dropped on PostgreSQL and migration fails

[Django]

#34027: When ForeignKey target field type is changed from CharField to
IntegerField, varchar_pattern_ops index is not dropped on PostgreSQL and
migration fails
+-
 Reporter:  Chris   |Owner:  David Sanders
 Type:  Bug |   Status:  assigned
Component:  Migrations  |  Version:  4.1
 Severity:  Normal  |   Resolution:
 Keywords:  PostgreSQL  | Triage Stage:  Unreviewed
Has patch:  1   |  Needs documentation:  0
  Needs tests:  0   |  Patch needs improvement:  0
Easy pickings:  0   |UI/UX:  0
+-
Changes (by David Sanders):

 * has_patch:  0 => 1


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Re: [Django] #34027: When ForeignKey target field type is changed from CharField to IntegerField, varchar_pattern_ops index is not dropped on PostgreSQL and migration fails

[Django]

#34027: When ForeignKey target field type is changed from CharField to
IntegerField, varchar_pattern_ops index is not dropped on PostgreSQL and
migration fails
+-
 Reporter:  Chris   |Owner:  David Sanders
 Type:  Bug |   Status:  assigned
Component:  Migrations  |  Version:  4.1
 Severity:  Normal  |   Resolution:
 Keywords:  PostgreSQL  | Triage Stage:  Unreviewed
Has patch:  0   |  Needs documentation:  0
  Needs tests:  0   |  Patch needs improvement:  0
Easy pickings:  0   |UI/UX:  0
+-

Comment (by David Sanders):

 Draft PR with failing test: https://github.com/django/django/pull/16087

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Re: [Django] #34027: When ForeignKey target field type is changed from CharField to IntegerField, varchar_pattern_ops index is not dropped on PostgreSQL and migration fails

[Django]

#34027: When ForeignKey target field type is changed from CharField to
IntegerField, varchar_pattern_ops index is not dropped on PostgreSQL and
migration fails
+-
 Reporter:  Chris   |Owner:  David Sanders
 Type:  Bug |   Status:  assigned
Component:  Migrations  |  Version:  4.1
 Severity:  Normal  |   Resolution:
 Keywords:  PostgreSQL  | Triage Stage:  Unreviewed
Has patch:  0   |  Needs documentation:  0
  Needs tests:  0   |  Patch needs improvement:  0
Easy pickings:  0   |UI/UX:  0
+-
Changes (by David Sanders):

 * owner:  nobody => David Sanders
 * status:  new => assigned
 * version:  3.2 => 4.1


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Ticket URL: 
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[Django] #34027: When ForeignKey target field type is changed from CharField to IntegerField, varchar_pattern_ops index is not dropped on PostgreSQL and migration fails

[Django]

#34027: When ForeignKey target field type is changed from CharField to
IntegerField, varchar_pattern_ops index is not dropped on PostgreSQL and
migration fails
--+
   Reporter:  Chris   |  Owner:  nobody
   Type:  Bug | Status:  new
  Component:  Migrations  |Version:  3.2
   Severity:  Normal  |   Keywords:  PostgreSQL
   Triage Stage:  Unreviewed  |  Has patch:  0
Needs documentation:  0   |Needs tests:  0
Patch needs improvement:  0   |  Easy pickings:  0
  UI/UX:  0   |
--+
 When a `ForeignKey` points to `CharField`, and the `CharField` is then
 changed to e.g. an `AutoField`, the migration fails with:

 {{{psycopg2.errors.DatatypeMismatch: operator class "varchar_pattern_ops"
 does not accept data type bigint}}}

 The reason seems to be that the original `ForeignKey` column in the
 database was a `varchar`, and Django created a `varchar_pattern_ops` index
 for it (e.g. one of those `..._like` indexes). Changing the column type to
 `bigint` is not accepted by this index.

 Manually dropping the index before attempting to run the migrations
 results in the migration being performed successfully.

 Performing the same set of migrations on SQLite and on MySQL does not
 result in an error, it only fails on PostgreSQL.


 

 Test case:
 - Use models.py:
 {{{
 class ModelA(models.Model):
 # Custom CharField primary key
 id = models.CharField(max_length=10, primary_key=True)

 class ModelB(models.Model):
 modela = models.ForeignKey(ModelA, on_delete=models.CASCADE)
 }}}
 - Run `makemigrations` and `migrate` commands
 - Alter `ModelA`:
 {{{
 class ModelA(models.Model):
 # No more custom primary key
 pass
 }}}
 - Run `makemigrations` and `migrate` commands again
 - Observe error on PostgreSQL, success on SQLite and MySQL
 - Manually `DROP` the `_modelb_modela_id_<...>_like` index in
 PostgreSQL
 - Again run the `migrate` command
 - Now the migration succeeds

 

 The problem is apparently that the
 `BaseDatabaseSchemaEditor._alter_field()` method simply calls
 `_alter_column_type_sql()` or `_alter_column_collation_sql()` to directly
 change the column type of relations pointing to the field. These methods
 do not generate any additional SQL related to indexes etc as the normal
 (PostgreSQL version of) `_alter_field` method would under similar
 circumstances. Note that the `..._like` index for the primary key itself
 _is_ dropped.

 Similarly also, if the primary key field is changed in the other
 direction, e.g. from `AutoField` to `CharField`, the `..._like` index is
 not added to the existing `ForeignKey`, it is only added for the primary
 key field, as it normally is when an integer field is changed to a text
 field.

 Possibly related issues: #27338 #27860

 Reported for version 3.2 but also observed on 4.1.

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