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

1 files changed, 31 insertions, 0 deletions

diff --git a/app/models/deployment.rb b/app/models/deployment.rb
index 1e338889714..183d538a867 100644
--- a/app/models/deployment.rb
+++ b/app/models/deployment.rb
@@ -42,4 +42,35 @@ class Deployment < ActiveRecord::Base
 
     project.repository.is_ancestor?(commit.id, sha)
   end
+
+
+  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
+
+      # 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).
+        update_all(first_deployed_to_production_at: self.created_at)
+    end
+  end
+
+  def previous_deployment
+    @previous_deployment ||=
+      project.deployments.joins(:environment).
+      where(environments: { name: self.environment.name }, ref: self.ref).
+      where.not(id: self.id).
+      take
+  end
 end