Implement review comments from @yorickpeterse

1. Change multiple updates to a single `update_all`

2. Use cascading deletes

3. Extract an average function for the database median.

4. Move database median to `lib/gitlab/database`

5. Use `delete_all` instead of `destroy_all`

6. Minor refactoring

4 files changed, 9 insertions, 9 deletions

diff --git a/db/fixtures/development/17_cycle_analytics.rb b/db/fixtures/development/17_cycle_analytics.rb
index f51d28d9ebf..e882a492757 100644
--- a/db/fixtures/development/17_cycle_analytics.rb
+++ b/db/fixtures/development/17_cycle_analytics.rb
@@ -4,7 +4,7 @@ require './spec/support/test_env'
 class Gitlab::Seeder::CycleAnalytics
   def initialize(project, perf: false)
     @project = project
-    @user = User.find(1)
+    @user = User.order(:id).last
     @issue_count = perf ? 1000 : 5
     stub_git_pre_receive!
   end
diff --git a/db/migrate/20160824124900_add_table_issue_metrics.rb b/db/migrate/20160824124900_add_table_issue_metrics.rb
index 04dff26043d..3d296c7e884 100644
--- a/db/migrate/20160824124900_add_table_issue_metrics.rb
+++ b/db/migrate/20160824124900_add_table_issue_metrics.rb
@@ -5,12 +5,12 @@ class AddTableIssueMetrics < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
   # Set this constant to true if this migration requires downtime.
-  DOWNTIME = false
+  DOWNTIME = true
 
   # 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 = ''
+  DOWNTIME_REASON = 'Adding foreign key'
 
   # 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
@@ -25,7 +25,7 @@ class AddTableIssueMetrics < ActiveRecord::Migration
 
   def change
     create_table :issue_metrics do |t|
-      t.references :issue, index: { name: "index_issue_metrics" }, foreign_key: true, null: false
+      t.references :issue, index: { name: "index_issue_metrics" }, foreign_key: true, dependent: :delete, null: false
 
       t.datetime 'first_mentioned_in_commit_at'
       t.datetime 'first_associated_with_milestone_at'
diff --git a/db/migrate/20160825052008_add_table_merge_request_metrics.rb b/db/migrate/20160825052008_add_table_merge_request_metrics.rb
index fb5494c5e8b..d743134d9a2 100644
--- a/db/migrate/20160825052008_add_table_merge_request_metrics.rb
+++ b/db/migrate/20160825052008_add_table_merge_request_metrics.rb
@@ -5,12 +5,12 @@ class AddTableMergeRequestMetrics < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
   # Set this constant to true if this migration requires downtime.
-  DOWNTIME = false
+  DOWNTIME = true
 
   # 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 = ''
+  DOWNTIME_REASON = 'Adding foreign key'
 
   # 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
@@ -25,7 +25,7 @@ class AddTableMergeRequestMetrics < ActiveRecord::Migration
 
   def change
     create_table :merge_request_metrics do |t|
-      t.references :merge_request, index: { name: "index_merge_request_metrics" }, foreign_key: true, null: false
+      t.references :merge_request, index: { name: "index_merge_request_metrics" }, foreign_key: true, dependent: :delete, null: false
 
       t.datetime 'latest_build_started_at'
       t.datetime 'latest_build_finished_at'
diff --git a/db/migrate/20160915042921_create_merge_requests_closing_issues.rb b/db/migrate/20160915042921_create_merge_requests_closing_issues.rb
index 02c28cbe0dd..049340f7085 100644
--- a/db/migrate/20160915042921_create_merge_requests_closing_issues.rb
+++ b/db/migrate/20160915042921_create_merge_requests_closing_issues.rb
@@ -25,8 +25,8 @@ class CreateMergeRequestsClosingIssues < ActiveRecord::Migration
 
   def change
     create_table :merge_requests_closing_issues do |t|
-      t.references :merge_request, foreign_key: true, index: true, null: false
-      t.references :issue, foreign_key: true, index: true, null: false
+      t.references :merge_request, foreign_key: true, index: true, dependent: :delete, null: false
+      t.references :issue, foreign_key: true, index: true, dependent: :delete, null: false
 
       t.timestamps null: false
     end