Adds uniqueness and a db index to the MergedMRN.mrn column

We query this column a lot via MRN so add a db index.
If there are duplicate rows with the same MRN something wrong has
happened so enforce uniqueness.

elcid/migrations/0067_auto_20221209_1453.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.16 on 2022-12-09 14:53
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('elcid', '0066_auto_20221201_1527'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='mergedmrn',
+            name='mrn',
+            field=models.CharField(db_index=True, max_length=256, unique=True),
+        ),
+    ]

elcid/models.py

@@ -35,7 +35,7 @@ class MergedMRN(models.Model):
     Patient 123 would have a patient merge object with MRN 77456
     """
     patient = models.ForeignKey(Patient, on_delete=models.CASCADE)
-    mrn = models.CharField(max_length=256)
+    mrn = models.CharField(max_length=256, unique=True, db_index=True)
     merge_comments = models.TextField(blank=True, null=True, default="")
     upstream_merge_datetime = models.DateTimeField(blank=True, null=True)
     our_merge_datetime = models.DateTimeField(blank=True, null=True)