Add existence checks before creating the timelogs table and time_estimate columns

Signed-off-by: Rémy Coutable <remy@rymai.me>

db/migrate/20161030005533_add_estimate_to_issuables.rb

-# See http://doc.gitlab.com/ce/development/migration_style_guide.html
-# for more information on how to write migrations for GitLab.
-
 class AddEstimateToIssuables < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
-  # Set this constant to true if this migration requires downtime.
   DOWNTIME = false
 
-  # When a migration requires downtime you **must** uncomment the following
-  # constant and define a short and easy to understand explanation as to why the
-  # migration requires downtime.
-  # DOWNTIME_REASON = ''
-
-  # When using the methods "add_concurrent_index" or "add_column_with_default"
-  # you must disable the use of transactions as these methods can not run in an
-  # existing transaction. When using "add_concurrent_index" make sure that this
-  # method is the _only_ method called in the migration, any other changes
-  # should go in a separate migration. This ensures that upon failure _only_ the
-  # index creation fails and can be retried or reverted easily.
-  #
-  # To disable transactions uncomment the following line and remove these
-  # comments:
-  # disable_ddl_transaction!
-
   def up
+    unless column_exists?(:issues, :time_estimate)
       add_column :issues, :time_estimate, :integer
+    end
+
+    unless column_exists?(:merge_requests, :time_estimate)
       add_column :merge_requests, :time_estimate, :integer
     end
+  end
 
   def down
+    if column_exists?(:issues, :time_estimate)
       remove_column :issues, :time_estimate
+    end
+
+    if column_exists?(:merge_requests, :time_estimate)
       remove_column :merge_requests, :time_estimate
     end
+  end
 end

db/migrate/20161030020610_create_timelogs.rb

@@ -3,7 +3,8 @@ class CreateTimelogs < ActiveRecord::Migration
 
   DOWNTIME = false
 
-  def change
+  def up
+    unless table_exists?(:timelogs)
       create_table :timelogs do |t|
         t.integer :time_spent, null: false
         t.references :trackable, polymorphic: true
@@ -15,4 +16,9 @@ class CreateTimelogs < ActiveRecord::Migration
       add_index :timelogs, [:trackable_type, :trackable_id]
       add_index :timelogs, :user_id
     end
+  end
+
+  def down
+    drop_table :timelogs if table_exists?(:timelogs)
+  end
 end