add unique index for role overrides

refs #CORE-1717

Change-Id: I8d9df475430e5758e5f952cfa13f81d64e40b78f
Reviewed-on: https://gerrit.instructure.com/160707
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>

db/migrate/20180814153129_add_unique_index_to_role_overrides.rb

@@ -0,0 +1,24 @@
+class AddUniqueIndexToRoleOverrides < ActiveRecord::Migration[5.1]
+  tag :postdeploy
+  disable_ddl_transaction!
+
+  def up
+    Account.find_ids_in_ranges(:batch_size => 100) do |min_id, max_id|
+      account_ids = Account.where(:id => min_id..max_id).pluck(:id)
+      dups = RoleOverride.where(:context_type => "Account", :context_id => account_ids).
+        group(:context_id, :permission, :role_id).having("COUNT(*) > 1").pluck(:context_id, :permission, :role_id)
+      dups.each do |account_id, permission, role_id|
+        RoleOverride.where(:context_type => "Account", :context_id => account_id, :permission => permission, :role_id => role_id).order(:id).offset(1).delete_all
+      end
+    end
+
+    add_index :role_overrides, [:context_id, :context_type, :role_id, :permission], unique: true, algorithm: :concurrently,
+      name: "index_role_overrides_on_context_role_permission"
+    remove_index :role_overrides, [:context_id, :context_type]
+  end
+
+  def down
+    remove_index :role_overrides, name: "index_role_overrides_on_context_role_permission"
+    add_index :role_overrides, [:context_id, :context_type]
+  end
+end