diff --git a/app/models/milestone.rb b/app/models/milestone.rb
index d05dcfd083a61f8c89bf70a164c9141c1697615d..14cc12b38a57bd362da4ef12976bff2a9886ba6e 100644
--- a/app/models/milestone.rb
+++ b/app/models/milestone.rb
@@ -131,9 +131,10 @@ class Milestone < ActiveRecord::Base
       rel.order(:project_id, :due_date).select('DISTINCT ON (project_id) id')
     else
       rel
-        .group(:project_id)
+        .group(:project_id, :due_date, :id)
         .having('due_date = MIN(due_date)')
         .pluck(:id, :project_id, :due_date)
+        .uniq(&:second)
         .map(&:first)
     end
   end
diff --git a/changelogs/unreleased/blackst0ne-rails5-activerecord-statementinvalid-mysql2-error-expression-1-of-select-list-is-not-in-group-by-clause.yml b/changelogs/unreleased/blackst0ne-rails5-activerecord-statementinvalid-mysql2-error-expression-1-of-select-list-is-not-in-group-by-clause.yml
new file mode 100644
index 0000000000000000000000000000000000000000..d9cccc4983072a28dcfa572839788a0f3aa9f2e0
--- /dev/null
+++ b/changelogs/unreleased/blackst0ne-rails5-activerecord-statementinvalid-mysql2-error-expression-1-of-select-list-is-not-in-group-by-clause.yml
@@ -0,0 +1,5 @@
+---
+title: "[Rails5] Fix milestone GROUP BY query"
+merge_request: 20256
+author: "@blackst0ne"
+type: fixed