old_contents: # -*- coding: utf-8 -*-
  
  import ujson
  
  from collections import defaultdict
  from django.conf import settings
  from django.db import migrations, models
  from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
  from django.db.migrations.state import StateApps
  from typing import Any, Dict
  
  def realm_emoji_name_to_id(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
      Reaction = apps.get_model('zerver', 'Reaction')
      RealmEmoji = apps.get_model('zerver', 'RealmEmoji')
      realm_emoji_by_realm_id = defaultdict(dict)   # type: Dict[int, Dict[str, Any]]
      for realm_emoji in RealmEmoji.objects.all():
          realm_emoji_by_realm_id[realm_emoji.realm_id] = {
              'id': str(realm_emoji.id),
              'name': realm_emoji.name,
              'deactivated': realm_emoji.deactivated,
          }
      for reaction in Reaction.objects.filter(reaction_type='realm_emoji'):
          realm_id = reaction.user_profile.realm_id
          emoji_name = reaction.emoji_name
          if realm_id in realm_emoji_by_realm_id:
              realm_emoji = realm_emoji_by_realm_id[realm_id].get(emoji_name)
              if realm_emoji is None:
                  # Realm emoji used in this reaction has been deleted so this
                  # reaction should also be deleted. We don't need to reverse
                  # this step in migration reversal code.
                  reaction.delete()
              else:
                  reaction.emoji_code = realm_emoji["id"]
                  reaction.save()
  
  def reversal(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
      Reaction = apps.get_model('zerver', 'Reaction')
      for reaction in Reaction.objects.filter(reaction_type='realm_emoji'):
          reaction.emoji_code = reaction.emoji_name
          reaction.save()
  
  class Migration(migrations.Migration):
  
      dependencies = [
          ('zerver', '0144_remove_realm_create_generic_bot_by_admins_only'),
      ]
  
      operations = [
          migrations.RunPython(realm_emoji_name_to_id,
                               reverse_code=reversal),
      ]
  
new_contents: # -*- coding: utf-8 -*-
  
  import ujson
  
  from collections import defaultdict
  from django.conf import settings
  from django.db import migrations, models
  from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
  from django.db.migrations.state import StateApps
  from typing import Any, Dict
  
  def realm_emoji_name_to_id(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
      Reaction = apps.get_model('zerver', 'Reaction')
      RealmEmoji = apps.get_model('zerver', 'RealmEmoji')
      realm_emoji_by_realm_id = defaultdict(dict)   # type: Dict[int, Dict[str, Any]]
      for realm_emoji in RealmEmoji.objects.all():
          realm_emoji_by_realm_id[realm_emoji.realm_id] = {
              'id': str(realm_emoji.id),
              'name': realm_emoji.name,
              'deactivated': realm_emoji.deactivated,
          }
      for reaction in Reaction.objects.filter(reaction_type='realm_emoji'):
          realm_id = reaction.user_profile.realm_id
          emoji_name = reaction.emoji_name
          realm_emoji = realm_emoji_by_realm_id.get(realm_id, {}).get(emoji_name)
          if realm_emoji is None:
              # Realm emoji used in this reaction has been deleted so this
              # reaction should also be deleted. We don't need to reverse
              # this step in migration reversal code.
              reaction.delete()
          else:
              reaction.emoji_code = realm_emoji["id"]
              reaction.save()
  
  def reversal(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
      Reaction = apps.get_model('zerver', 'Reaction')
      for reaction in Reaction.objects.filter(reaction_type='realm_emoji'):
          reaction.emoji_code = reaction.emoji_name
          reaction.save()
  
  class Migration(migrations.Migration):
  
      dependencies = [
          ('zerver', '0144_remove_realm_create_generic_bot_by_admins_only'),
      ]
  
      operations = [
          migrations.RunPython(realm_emoji_name_to_id,
                               reverse_code=reversal),
      ]
  
subject: Fix migration to correctly handle corner case.