refactor: rename fields from "key" to "_key"

We're doing this because "key" is a reserved keyword for MySQL. While
Django will escape this for us automatically, we've historically seen
downstream tooling break when column names use reserved keywords. Also,
it makes things harder for people who are querying the database directly
and might not realize that they need to esacape column names.

In any event, this has caused enough pain in the past to warrant a check
in edx-platform's CI, where it checks column names against a whole list
of reserved keywords for several systems:

https://github.com/openedx/edx-django-release-util/blob/master/release_util/management/commands/default_reserved_keywords.yml

It's likely that we'll want rename at least some of these columns later,
but this lets us introduce them into the system without worrying that
they'll break anything.

openedx_learning/core/components/migrations/0002_alter_componentversioncontent_key.py

@@ -0,0 +1,20 @@
+# Generated by Django 4.2.10 on 2024-02-14 22:02
+
+from django.db import migrations
+
+import openedx_learning.lib.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('oel_components', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='componentversioncontent',
+            name='key',
+            field=openedx_learning.lib.fields.MultiCollationCharField(db_collations={'mysql': 'utf8mb4_bin', 'sqlite': 'BINARY'}, db_column='_key', max_length=500),
+        ),
+    ]

openedx_learning/core/components/models.py

@@ -237,7 +237,13 @@ class ComponentVersionContent(models.Model):
     content = models.ForeignKey(Content, on_delete=models.RESTRICT)
 
     uuid = immutable_uuid_field()
-    key = key_field()
+
+    # "key" is a reserved word for MySQL, so we're temporarily using the column
+    # name of "_key" to avoid breaking downstream tooling. A possible
+    # alternative name for this would be "path", since it's most often used as
+    # an internal file path. However, we might also want to put special
+    # identifiers that don't map as cleanly to file paths at some point.
+    key = key_field(db_column="_key")
 
     # Long explanation for the ``learner_downloadable`` field:
     #

openedx_learning/core/publishing/migrations/0002_alter_learningpackage_key_and_more.py

@@ -0,0 +1,25 @@
+# Generated by Django 4.2.10 on 2024-02-14 22:02
+
+from django.db import migrations
+
+import openedx_learning.lib.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('oel_publishing', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='learningpackage',
+            name='key',
+            field=openedx_learning.lib.fields.MultiCollationCharField(db_collations={'mysql': 'utf8mb4_bin', 'sqlite': 'BINARY'}, db_column='_key', max_length=500),
+        ),
+        migrations.AlterField(
+            model_name='publishableentity',
+            name='key',
+            field=openedx_learning.lib.fields.MultiCollationCharField(db_collations={'mysql': 'utf8mb4_bin', 'sqlite': 'BINARY'}, db_column='_key', max_length=500),
+        ),
+    ]

openedx_learning/core/publishing/models.py

@@ -38,7 +38,14 @@ class LearningPackage(models.Model):  # type: ignore[django-manager-missing]
     id = models.AutoField(primary_key=True)
 
     uuid = immutable_uuid_field()
-    key = key_field()
+
+    # "key" is a reserved word for MySQL, so we're temporarily using the column
+    # name of "_key" to avoid breaking downstream tooling. There's an open
+    # question as to whether this field needs to exist at all, or whether the
+    # top level library key it's currently used for should be entirely in the
+    # LibraryContent model.
+    key = key_field(db_column="_key")
+
     title = case_insensitive_char_field(max_length=500, blank=False)
 
     # TODO: We should probably defer this field, since many things pull back
@@ -172,7 +179,12 @@ class PublishableEntity(models.Model):
         on_delete=models.CASCADE,
         related_name="publishable_entities",
     )
-    key = key_field()
+
+    # "key" is a reserved word for MySQL, so we're temporarily using the column
+    # name of "_key" to avoid breaking downstream tooling. Consider renaming
+    # this later.
+    key = key_field(db_column="_key")
+
     created = manual_date_time_field()
     created_by = models.ForeignKey(
         settings.AUTH_USER_MODEL,

openedx_learning/lib/fields.py

@@ -109,7 +109,7 @@ def immutable_uuid_field() -> models.UUIDField:
     )
 
 
-def key_field() -> MultiCollationCharField:
+def key_field(**kwargs) -> MultiCollationCharField:
     """
     Externally created Identifier fields.
 
@@ -120,7 +120,7 @@ def key_field() -> MultiCollationCharField:
     Other apps should *not* make references to these values directly, since
     these values may in theory change (even if this is rare in practice).
     """
-    return case_sensitive_char_field(max_length=500, blank=False)
+    return case_sensitive_char_field(max_length=500, blank=False, **kwargs)
 
 
 def hash_field() -> models.CharField: