Use migration classes in migrations where current definition conflicts with older (#26390)

db/migrate/20181116173541_copy_account_stats.rb

@@ -3,6 +3,10 @@
 class CopyAccountStats < ActiveRecord::Migration[5.2]
   disable_ddl_transaction!
 
+  class MigrationAccount < ApplicationRecord
+    self.table_name = :accounts
+  end
+
   def up
     safety_assured do
       if supports_upsert?
@@ -27,7 +31,7 @@ class CopyAccountStats < ActiveRecord::Migration[5.2]
   def up_fast
     say 'Upsert is available, importing counters using the fast method'
 
-    Account.unscoped.select('id').find_in_batches(batch_size: 5_000) do |accounts|
+    MigrationAccount.unscoped.select('id').find_in_batches(batch_size: 5_000) do |accounts|
       execute <<-SQL.squish
         INSERT INTO account_stats (account_id, statuses_count, following_count, followers_count, created_at, updated_at)
         SELECT id, statuses_count, following_count, followers_count, created_at, updated_at
@@ -44,7 +48,7 @@ class CopyAccountStats < ActiveRecord::Migration[5.2]
 
     # We cannot use bulk INSERT or overarching transactions here because of possible
     # uniqueness violations that we need to skip over
-    Account.unscoped.select('id, statuses_count, following_count, followers_count, created_at, updated_at').find_each do |account|
+    MigrationAccount.unscoped.select('id, statuses_count, following_count, followers_count, created_at, updated_at').find_each do |account|
       params = [account.id, account[:statuses_count], account[:following_count], account[:followers_count], account.created_at, account.updated_at]
       exec_insert('INSERT INTO account_stats (account_id, statuses_count, following_count, followers_count, created_at, updated_at) VALUES ($1, $2, $3, $4, $5, $6)', nil, params)
     rescue ActiveRecord::RecordNotUnique