Avoid doublons on transcriptions by adding a unique constraint

arkindex/documents/migrations/0025_avoid_doublons.py

+# Generated by Django 2.1 on 2018-09-17 08:35
+
+from django.db import migrations, models
+from django.db import connection
+import django.db.models.deletion
+
+# This sql query delete all the transcriptions
+# that have the same (element_id, zone_id, text)
+# keeping only the first item
+SQL_REMOVE = '''
+delete from documents_transcription
+where id in (
+    select unnest((array_agg(id))[2:])
+    from documents_transcription
+    group by (element_id, zone_id, text)
+    having count(*) > 1
+);
+'''
+
+# This sql query create a unique together index
+# on (element_id, zone_id, text)
+# It cannot be automatically achieved by Django
+# as it needs to make a md5 hash of text to fit in its index max size
+SQL_UNIQUE = '''
+create unique index documents_transcription_uniq_elt_zone_text
+on documents_transcription (
+    zone_id,
+    element_id,
+    md5(text)
+)
+'''
+
+
+def remove_doublons(apps, schema_editor):
+    '''
+    Run above sql queries
+    '''
+    with connection.cursor() as cursor:
+        cursor.execute(SQL_REMOVE)
+        cursor.execute(SQL_UNIQUE)
+
+    # Custom index
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('images', '0005_zone_polygon_tmp'),
+        ('documents', '0024_page_text'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='elementpath',
+            name='element',
+            field=models.ForeignKey(
+                on_delete=django.db.models.deletion.CASCADE,
+                related_name='paths',
+                to='documents.Element',
+            ),
+        ),
+        migrations.RunPython(remove_doublons),
+    ]

arkindex/documents/models.py

@@ -474,6 +474,14 @@ class Transcription(models.Model):
     text = models.TextField(null=True, blank=True)
     score = models.FloatField(null=True, blank=True)
 
+    class Meta:
+        # This index is manually created in a migration to
+        # support the md5(text) operation
+        # unique_together = (
+        #     ('element', 'zone', 'text')
+        # )
+        pass
+
     def __str__(self):
         return 'Transcription: {}'.format(self.text[:20])