Implement (some) comments from @DouweM's review.

- Move things common to `Issue` and `MergeRequest` into `Issuable`
- Move more database-specific functions into `Gitlab::Database`
- Indentation changes and other minor refactorings.

app/models/concerns/issuable.rb

@@ -33,6 +33,8 @@ module Issuable
     has_many :labels, through: :label_links
     has_many :todos, as: :target, dependent: :destroy
 
+    has_one :metrics
+
     validates :author, presence: true
     validates :title, presence: true, length: { within: 0..255 }
 
@@ -82,6 +84,7 @@ module Issuable
     acts_as_paranoid
 
     after_save :update_assignee_cache_counts, if: :assignee_id_changed?
+    after_save :record_metrics
 
     def update_assignee_cache_counts
       # make sure we flush the cache for both the old *and* new assignee
@@ -287,4 +290,9 @@ module Issuable
   def can_move?(*)
     false
   end
+
+  def record_metrics
+    metrics = self.metrics || create_metrics
+    metrics.record!
+  end
 end

app/models/cycle_analytics.rb

 class CycleAnalytics
   include Gitlab::Database::Median
+  include Gitlab::Database::DateTime
 
   def initialize(project, from:)
     @project = project
@@ -12,58 +13,64 @@ class CycleAnalytics
   end
 
   def issue
-    calculate_metric!(:issue,
-                      TableReferences.issues[:created_at],
-                      [TableReferences.issue_metrics[:first_associated_with_milestone_at],
-                       TableReferences.issue_metrics[:first_added_to_board_at]])
+    calculate_metric(:issue,
+                     TableReferences.issues[:created_at],
+                     [TableReferences.issue_metrics[:first_associated_with_milestone_at],
+                      TableReferences.issue_metrics[:first_added_to_board_at]])
   end
 
   def plan
-    calculate_metric!(:plan,
-                      [TableReferences.issue_metrics[:first_associated_with_milestone_at],
-                       TableReferences.issue_metrics[:first_added_to_board_at]],
-                      TableReferences.issue_metrics[:first_mentioned_in_commit_at])
+    calculate_metric(:plan,
+                     [TableReferences.issue_metrics[:first_associated_with_milestone_at],
+                      TableReferences.issue_metrics[:first_added_to_board_at]],
+                     TableReferences.issue_metrics[:first_mentioned_in_commit_at])
   end
 
   def code
-    calculate_metric!(:code,
-                      TableReferences.issue_metrics[:first_mentioned_in_commit_at],
-                      TableReferences.merge_requests[:created_at])
+    calculate_metric(:code,
+                     TableReferences.issue_metrics[:first_mentioned_in_commit_at],
+                     TableReferences.merge_requests[:created_at])
   end
 
   def test
-    calculate_metric!(:test,
-                      TableReferences.merge_request_metrics[:latest_build_started_at],
-                      TableReferences.merge_request_metrics[:latest_build_finished_at])
+    calculate_metric(:test,
+                     TableReferences.merge_request_metrics[:latest_build_started_at],
+                     TableReferences.merge_request_metrics[:latest_build_finished_at])
   end
 
   def review
-    calculate_metric!(:review,
-                      TableReferences.merge_requests[:created_at],
-                      TableReferences.merge_request_metrics[:merged_at])
+    calculate_metric(:review,
+                     TableReferences.merge_requests[:created_at],
+                     TableReferences.merge_request_metrics[:merged_at])
   end
 
   def staging
-    calculate_metric!(:staging,
-                      TableReferences.merge_request_metrics[:merged_at],
-                      TableReferences.merge_request_metrics[:first_deployed_to_production_at])
+    calculate_metric(:staging,
+                     TableReferences.merge_request_metrics[:merged_at],
+                     TableReferences.merge_request_metrics[:first_deployed_to_production_at])
   end
 
   def production
-    calculate_metric!(:production,
-                      TableReferences.issues[:created_at],
-                      TableReferences.merge_request_metrics[:first_deployed_to_production_at])
+    calculate_metric(:production,
+                     TableReferences.issues[:created_at],
+                     TableReferences.merge_request_metrics[:first_deployed_to_production_at])
   end
 
   private
 
-  def calculate_metric!(name, start_time_attrs, end_time_attrs)
+  def calculate_metric(name, start_time_attrs, end_time_attrs)
     cte_table = Arel::Table.new("cte_table_for_#{name}")
 
-    # Add a `SELECT` for (end_time - start-time), and add an alias for it.
-    query = Arel::Nodes::As.new(cte_table, subtract_datetimes(base_query, end_time_attrs, start_time_attrs, name.to_s))
-    queries = Array.wrap(median_datetime(cte_table, query, name))
-    results = queries.map { |query| run_query(query) }
+    # Build a `SELECT` query. We find the first of the `end_time_attrs` that isn't `NULL` (call this end_time).
+    # Next, we find the first of the start_time_attrs that isn't `NULL` (call this start_time).
+    # We compute the (end_time - start_time) interval, and give it an alias based on the current
+    # cycle analytics stage.
+    interval_query = Arel::Nodes::As.new(
+      cte_table,
+      subtract_datetimes(base_query, end_time_attrs, start_time_attrs, name.to_s))
+
+    median_queries = Array.wrap(median_datetime(cte_table, interval_query, name))
+    results = median_queries.map { |query| run_query(query) }
     extract_median(results).presence
   end
 
@@ -82,51 +89,17 @@ class CycleAnalytics
             where(TableReferences.issues[:created_at].gteq(@from))
 
     # Load merge_requests
-    query = query.join(TableReferences.merge_requests, Arel::Nodes::OuterJoin).on(TableReferences.merge_requests[:id].eq(arel_table[:merge_request_id])).
-            join(TableReferences.merge_request_metrics).on(TableReferences.merge_requests[:id].eq(TableReferences.merge_request_metrics[:merge_request_id]))
+    query = query.join(TableReferences.merge_requests, Arel::Nodes::OuterJoin).
+            on(TableReferences.merge_requests[:id].eq(arel_table[:merge_request_id])).
+            join(TableReferences.merge_request_metrics).
+            on(TableReferences.merge_requests[:id].eq(TableReferences.merge_request_metrics[:merge_request_id]))
 
     # Limit to merge requests that have been deployed to production after `@from`
     query.where(TableReferences.merge_request_metrics[:first_deployed_to_production_at].gteq(@from))
   end
 
-  # Note: We use COALESCE to pick up the first non-null column for end_time / start_time.
-  def subtract_datetimes(query_so_far, end_time_attrs, start_time_attrs, as)
-    diff_fn = case ActiveRecord::Base.connection.adapter_name
-              when 'PostgreSQL'
-                Arel::Nodes::Subtraction.new(
-                  Arel::Nodes::NamedFunction.new("COALESCE", Array.wrap(end_time_attrs)),
-                  Arel::Nodes::NamedFunction.new("COALESCE", Array.wrap(start_time_attrs)))
-              when 'Mysql2'
-                Arel::Nodes::NamedFunction.new(
-                  "TIMESTAMPDIFF",
-                  [Arel.sql('second'),
-                   Arel::Nodes::NamedFunction.new("COALESCE", Array.wrap(start_time_attrs)),
-                   Arel::Nodes::NamedFunction.new("COALESCE", Array.wrap(end_time_attrs))])
-              else
-                raise NotImplementedError, "Cycle analytics doesn't support your database type."
-              end
-
-    query_so_far.project(diff_fn.as(as))
-  end
-
   def run_query(query)
-    if query.is_a? String
-      ActiveRecord::Base.connection.execute query
-    else
-      ActiveRecord::Base.connection.execute query.to_sql
-    end
-  end
-
-  def extract_median(results)
-    result = results.compact.first
-
-    case ActiveRecord::Base.connection.adapter_name
-    when 'PostgreSQL'
-      result = result.first.presence
-      median = result['median'] if result
-      median.to_f if median
-    when 'Mysql2'
-      result.to_a.flatten.first
-    end
+    query = query.to_sql unless query.is_a?(String)
+    ActiveRecord::Base.connection.execute(query)
   end
 end

app/models/deployment.rb

@@ -46,22 +46,19 @@ class Deployment < ActiveRecord::Base
 
   def update_merge_request_metrics
     if environment.update_merge_request_metrics?
-      query = project.merge_requests.
-              joins(:metrics).
-              where(target_branch: self.ref, merge_request_metrics: { first_deployed_to_production_at: nil })
-
-      merge_requests =
-        if previous_deployment
-          query.where("merge_request_metrics.merged_at <= ? AND merge_request_metrics.merged_at >= ?",
-                      self.created_at,
-                      previous_deployment.created_at)
-        else
-          query.where("merge_request_metrics.merged_at <= ?", self.created_at)
-        end
+      merge_requests = project.merge_requests.
+                       joins(:metrics).
+                       where(target_branch: self.ref, merge_request_metrics: { first_deployed_to_production_at: nil }).
+                       where("merge_request_metrics.merged_at <= ?", self.created_at)
+
+      if previous_deployment
+        merge_requests = merge_requests.where("merge_request_metrics.merged_at >= ?", previous_deployment.created_at)
+      end
 
       # Need to use `map` instead of `select` because MySQL doesn't allow `SELECT`ing from the same table
       # that we're updating.
-      MergeRequest::Metrics.where(merge_request_id: merge_requests.map(&:id), first_deployed_to_production_at: nil).
+      MergeRequest::Metrics.
+        where(merge_request_id: merge_requests.map(&:id), first_deployed_to_production_at: nil).
         update_all(first_deployed_to_production_at: self.created_at)
     end
   end

app/models/issue.rb

@@ -23,8 +23,6 @@ class Issue < ActiveRecord::Base
 
   has_many :events, as: :target, dependent: :destroy
 
-  has_one :metrics
-
   has_many :merge_requests_closing_issues, class_name: 'MergeRequestsClosingIssues'
   has_many :closed_by_merge_requests, through: :merge_requests_closing_issues, source: :merge_request
 
@@ -41,8 +39,6 @@ class Issue < ActiveRecord::Base
   scope :order_due_date_asc, -> { reorder('issues.due_date IS NULL, issues.due_date ASC') }
   scope :order_due_date_desc, -> { reorder('issues.due_date IS NULL, issues.due_date DESC') }
 
-  after_save :record_metrics
-
   attr_spammable :title, spam_title: true
   attr_spammable :description, spam_description: true
 
@@ -277,9 +273,4 @@ class Issue < ActiveRecord::Base
   def check_for_spam?
     project.public?
   end
-
-  def record_metrics
-    metrics = Metrics.find_or_create_by(issue_id: self.id)
-    metrics.record!
-  end
 end

app/models/issue/metrics.rb

@@ -10,7 +10,7 @@ class Issue::Metrics < ActiveRecord::Base
       self.first_added_to_board_at = Time.now
     end
 
-    self.save if self.changed?
+    self.save
   end
 
   private

app/models/merge_request.rb

@@ -13,7 +13,6 @@ class MergeRequest < ActiveRecord::Base
   has_many :merge_request_diffs, dependent: :destroy
   has_one :merge_request_diff,
     -> { order('merge_request_diffs.id DESC') }
-  has_one :metrics
 
   has_many :events, as: :target, dependent: :destroy
 
@@ -35,8 +34,6 @@ class MergeRequest < ActiveRecord::Base
   # when creating new merge request
   attr_accessor :can_be_created, :compare_commits, :compare
 
-  after_save :record_metrics
-
   state_machine :state, initial: :opened do
     event :close do
       transition [:reopened, :opened] => :closed
@@ -515,7 +512,7 @@ class MergeRequest < ActiveRecord::Base
     transaction do
       self.merge_requests_closing_issues.delete_all
       closes_issues(current_user).each do |issue|
-        MergeRequestsClosingIssues.create!(merge_request: self, issue: issue)
+        self.merge_requests_closing_issues.create!(issue: issue)
       end
     end
   end
@@ -845,9 +842,4 @@ class MergeRequest < ActiveRecord::Base
       @conflicts_can_be_resolved_in_ui = false
     end
   end
-
-  def record_metrics
-    metrics = Metrics.find_or_create_by(merge_request_id: self.id)
-    metrics.record!
-  end
 end

app/models/merge_request/metrics.rb

@@ -6,6 +6,6 @@ class MergeRequest::Metrics < ActiveRecord::Base
       self.merged_at = Time.now
     end
 
-    self.save if self.changed?
+    self.save
   end
 end

config/routes.rb

@@ -780,7 +780,7 @@ Rails.application.routes.draw do
 
         resources :environments
 
-        resource :cycle_analytics
+        resource :cycle_analytics, only: [:show]
 
         resources :builds, only: [:index, :show], constraints: { id: /\d+/ } do
           collection do

lib/gitlab/database/date_time.rb

+module Gitlab
+  module Database
+    module DateTime
+      # Find the first of the `end_time_attrs` that isn't `NULL`. Subtract from it
+      # the first of the `start_time_attrs` that isn't NULL. `SELECT` the resulting interval
+      # along with an alias specified by the `as` parameter.
+      #
+      # Note: For MySQL, the interval is returned in seconds.
+      #       For PostgreSQL, the interval is returned as an INTERVAL type.
+      def subtract_datetimes(query_so_far, end_time_attrs, start_time_attrs, as)
+        diff_fn = if Gitlab::Database.postgresql?
+                    Arel::Nodes::Subtraction.new(
+                      Arel::Nodes::NamedFunction.new("COALESCE", Array.wrap(end_time_attrs)),
+                      Arel::Nodes::NamedFunction.new("COALESCE", Array.wrap(start_time_attrs)))
+                  elsif Gitlab::Database.mysql?
+                    Arel::Nodes::NamedFunction.new(
+                      "TIMESTAMPDIFF",
+                      [Arel.sql('second'),
+                       Arel::Nodes::NamedFunction.new("COALESCE", Array.wrap(start_time_attrs)),
+                       Arel::Nodes::NamedFunction.new("COALESCE", Array.wrap(end_time_attrs))])
+                  end
+
+        query_so_far.project(diff_fn.as(as))
+      end
+    end
+  end
+end

lib/gitlab/database/median.rb

@@ -3,13 +3,22 @@ module Gitlab
   module Database
     module Median
       def median_datetime(arel_table, query_so_far, column_sym)
-        case ActiveRecord::Base.connection.adapter_name
-        when 'PostgreSQL'
+        if Gitlab::Database.postgresql?
           pg_median_datetime(arel_table, query_so_far, column_sym)
-        when 'Mysql2'
+        elsif Gitlab::Database.mysql?
           mysql_median_datetime(arel_table, query_so_far, column_sym)
-        else
-          raise NotImplementedError, "We haven't implemented a database median strategy for your database type."
+        end
+      end
+
+      def extract_median(results)
+        result = results.compact.first
+
+        if Gitlab::Database.postgresql?
+          result = result.first.presence
+          median = result['median'] if result
+          median.to_f if median
+        elsif Gitlab::Database.mysql?
+          result.to_a.flatten.first
         end
       end
 
@@ -17,17 +26,23 @@ module Gitlab
         query = arel_table.
                 from(arel_table.project(Arel.sql('*')).order(arel_table[column_sym]).as(arel_table.table_name)).
                 project(average([arel_table[column_sym]], 'median')).
-                where(Arel::Nodes::Between.new(Arel.sql("(select @row_id := @row_id + 1)"),
-                                               Arel::Nodes::And.new([Arel.sql('@ct/2.0'),
-                                                                     Arel.sql('@ct/2.0 + 1')]))).
+                where(Arel::Nodes::Between.new(
+                       Arel.sql("(select @row_id := @row_id + 1)"),
+                       Arel::Nodes::And.new(
+                         [Arel.sql('@ct/2.0'),
+                          Arel.sql('@ct/2.0 + 1')]
+                       )
+                     )).
                 # Disallow negative values
                 where(arel_table[column_sym].gteq(0))
 
-        [Arel.sql("CREATE TEMPORARY TABLE IF NOT EXISTS #{query_so_far.to_sql}"),
-         Arel.sql("set @ct := (select count(1) from #{arel_table.table_name});"),
-         Arel.sql("set @row_id := 0;"),
-         query,
-         Arel.sql("DROP TEMPORARY TABLE IF EXISTS #{arel_table.table_name};")]
+        [
+          Arel.sql("CREATE TEMPORARY TABLE IF NOT EXISTS #{query_so_far.to_sql}"),
+          Arel.sql("set @ct := (select count(1) from #{arel_table.table_name});"),
+          Arel.sql("set @row_id := 0;"),
+          query,
+          Arel.sql("DROP TEMPORARY TABLE IF EXISTS #{arel_table.table_name};")
+        ]
       end
 
       def pg_median_datetime(arel_table, query_so_far, column_sym)
@@ -42,14 +57,15 @@ module Gitlab
         #           9 |      2 |  3
         #          15 |      3 |  3
         cte_table = Arel::Table.new("ordered_records")
-        cte = Arel::Nodes::As.new(cte_table,
-                                  arel_table.
-                                    project(arel_table[column_sym].as(column_sym.to_s),
-                                            Arel::Nodes::Over.new(Arel::Nodes::NamedFunction.new("row_number", []),
-                                                                  Arel::Nodes::Window.new.order(arel_table[column_sym])).as('row_id'),
-                                            arel_table.project("COUNT(1)").as('ct')).
-                                    # Disallow negative values
-                                    where(arel_table[column_sym].gteq(zero_interval)))
+        cte = Arel::Nodes::As.new(
+          cte_table,
+          arel_table.
+            project(arel_table[column_sym].as(column_sym.to_s),
+                    Arel::Nodes::Over.new(Arel::Nodes::NamedFunction.new("row_number", []),
+                                          Arel::Nodes::Window.new.order(arel_table[column_sym])).as('row_id'),
+                    arel_table.project("COUNT(1)").as('ct')).
+            # Disallow negative values
+            where(arel_table[column_sym].gteq(zero_interval)))
 
         # From the CTE, select either the middle row or the middle two rows (this is accomplished
         # by 'where cte.row_id between cte.ct / 2.0 AND cte.ct / 2.0 + 1'). Find the average of the