diff --git a/db/migrate/20180508100222_add_not_null_constraint_to_project_mirror_data_foreign_key.rb b/db/migrate/20180508100222_add_not_null_constraint_to_project_mirror_data_foreign_key.rb
new file mode 100644
index 00000000000..82087d15ccb
--- /dev/null
+++ b/db/migrate/20180508100222_add_not_null_constraint_to_project_mirror_data_foreign_key.rb
@@ -0,0 +1,21 @@
+class AddNotNullConstraintToProjectMirrorDataForeignKey < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  class ProjectImportState < ActiveRecord::Base
+    include EachBatch
+
+    self.table_name = 'project_mirror_data'
+  end
+
+  def up
+    ProjectImportState.where(project_id: nil).delete_all
+
+    change_column_null :project_mirror_data, :project_id, false
+  end
+
+  def down
+    change_column_null :project_mirror_data, :project_id, true
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 6fd10785d77..6f40f70d350 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180508055821) do
+ActiveRecord::Schema.define(version: 20180508100222) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1529,7 +1529,7 @@ ActiveRecord::Schema.define(version: 20180508055821) do
   add_index "project_import_data", ["project_id"], name: "index_project_import_data_on_project_id", using: :btree
 
   create_table "project_mirror_data", force: :cascade do |t|
-    t.integer "project_id"
+    t.integer "project_id", null: false
     t.string "status"
     t.string "jid"
     t.text "last_error"
diff --git a/spec/migrations/add_not_null_constraint_to_project_mirror_data_foreign_key_spec.rb b/spec/migrations/add_not_null_constraint_to_project_mirror_data_foreign_key_spec.rb
new file mode 100644
index 00000000000..6fd3cb1f44e
--- /dev/null
+++ b/spec/migrations/add_not_null_constraint_to_project_mirror_data_foreign_key_spec.rb
@@ -0,0 +1,18 @@
+require 'spec_helper'
+require Rails.root.join('db', 'migrate', '20180508100222_add_not_null_constraint_to_project_mirror_data_foreign_key.rb')
+
+describe AddNotNullConstraintToProjectMirrorDataForeignKey, :migration do
+  let(:namespaces) { table(:namespaces) }
+  let(:projects) { table(:projects) }
+  let(:import_state) { table(:project_mirror_data) }
+
+  before do
+    import_state.create!(id: 1, project_id: nil, status: :started)
+  end
+
+  it 'removes every import state without an associated project_id' do
+    expect do
+      subject.up
+    end.to change { import_state.count }.from(1).to(0)
+  end
+end