FIX: add a unique index to developers table (#8436)

db/migrate/20191128222140_add_unique_index_to_developers.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+class AddUniqueIndexToDevelopers < ActiveRecord::Migration[6.0]
+  def up
+    execute <<~SQL
+      DELETE FROM developers d1
+      USING (
+        SELECT MAX(id) as id, user_id
+        FROM developers
+        GROUP BY user_id
+        HAVING COUNT(*) > 1
+      ) d2
+      WHERE
+        d1.user_id = d2.user_id AND
+        d1.id <> d2.id
+    SQL
+
+    add_index :developers, %i(user_id), unique: true
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end