Add primary_answer field to AnatomyQuestion model and create migration to backfill existing values

anatomy/migrations/0026_add_primary_answer.py

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+"""Add primary_answer FK to AnatomyQuestion and backfill existing values.
+
+This migration adds a nullable FK `primary_answer` on AnatomyQuestion
+pointing to Answer, then backfills it using the current logic of
+selecting the first non-proposed CORRECT answer (status '2').
+"""
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+def forwards(apps, schema_editor):
+    AnatomyQuestion = apps.get_model('anatomy', 'AnatomyQuestion')
+    Answer = apps.get_model('anatomy', 'Answer')
+
+    # Select first non-proposed CORRECT answer (status '2') per question
+    for q in AnatomyQuestion.objects.all():
+        ans = Answer.objects.filter(question_id=q.pk, proposed=False, status='2').first()
+        if ans:
+            q.primary_answer_id = ans.pk
+            q.save(update_fields=['primary_answer'])
+
+
+def backwards(apps, schema_editor):
+    AnatomyQuestion = apps.get_model('anatomy', 'AnatomyQuestion')
+    for q in AnatomyQuestion.objects.all():
+        q.primary_answer = None
+        q.save(update_fields=['primary_answer'])
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('anatomy', '0025_exam_created_exam_updated'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='anatomyquestion',
+            name='primary_answer',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='primary_for_questions', to='anatomy.answer'),
+        ),
+        migrations.RunPython(forwards, backwards),
+    ]