Add a database view for postgres foreign keys

db/migrate/20210719145532_add_foreign_keys_view.rb

+# frozen_string_literal: true
+
+class AddForeignKeysView < ActiveRecord::Migration[6.1]
+  def up
+    execute(<<~SQL)
+      CREATE OR REPLACE VIEW postgres_foreign_keys AS
+      SELECT
+          pg_constraint.oid AS oid,
+          pg_constraint.conname AS name,
+          constrained_namespace.nspname::text || '.'::text || constrained_table.relname::text AS constrained_table_identifier,
+          referenced_namespace.nspname::text || '.'::text || referenced_table.relname::text AS referenced_table_identifier
+      FROM pg_constraint
+               INNER JOIN pg_class constrained_table ON constrained_table.oid = pg_constraint.conrelid
+               INNER JOIN pg_class referenced_table ON referenced_table.oid = pg_constraint.confrelid
+               INNER JOIN pg_namespace constrained_namespace ON constrained_table.relnamespace = constrained_namespace.oid
+               INNER JOIN pg_namespace referenced_namespace ON referenced_table.relnamespace = referenced_namespace.oid
+      WHERE contype = 'f';
+    SQL
+  end
+
+  def down
+    execute(<<~SQL)
+      DROP VIEW IF EXISTS postgres_foreign_keys
+    SQL
+  end
+end

db/schema_migrations/20210719145532

+5e088e5109b50d8f4fadd37a0382d7dc4ce856a851ec2b97f8d5d868c3cb19fd
\ No newline at end of file

db/structure.sql

@@ -16519,6 +16519,18 @@ CREATE SEQUENCE pool_repositories_id_seq
 
 ALTER SEQUENCE pool_repositories_id_seq OWNED BY pool_repositories.id;
 
+CREATE VIEW postgres_foreign_keys AS
+ SELECT pg_constraint.oid,
+    pg_constraint.conname AS name,
+    (((constrained_namespace.nspname)::text || '.'::text) || (constrained_table.relname)::text) AS constrained_table_identifier,
+    (((referenced_namespace.nspname)::text || '.'::text) || (referenced_table.relname)::text) AS referenced_table_identifier
+   FROM ((((pg_constraint
+     JOIN pg_class constrained_table ON ((constrained_table.oid = pg_constraint.conrelid)))
+     JOIN pg_class referenced_table ON ((referenced_table.oid = pg_constraint.confrelid)))
+     JOIN pg_namespace constrained_namespace ON ((constrained_table.relnamespace = constrained_namespace.oid)))
+     JOIN pg_namespace referenced_namespace ON ((referenced_table.relnamespace = referenced_namespace.oid)))
+  WHERE (pg_constraint.contype = 'f'::"char");
+
 CREATE VIEW postgres_index_bloat_estimates AS
  SELECT (((relation_stats.nspname)::text || '.'::text) || (relation_stats.idxname)::text) AS identifier,
     (

lib/gitlab/database/postgres_foreign_key.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Database
+    class PostgresForeignKey < ApplicationRecord
+      self.primary_key = :oid
+
+      scope :by_referenced_table_identifier, ->(identifier) do
+        raise ArgumentError, "Referenced table name is not fully qualified with a schema: #{identifier}" unless identifier =~ /^\w+\.\w+$/
+
+        where(referenced_table_identifier: identifier)
+      end
+    end
+  end
+end

spec/lib/gitlab/database/postgres_foreign_key_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Database::PostgresForeignKey, type: :model do
+  # PostgresForeignKey does not `behaves_like 'a postgres model'` because it does not correspond 1-1 with a single entry
+  # in pg_class
+
+  before do
+    ActiveRecord::Base.connection.execute(<<~SQL)
+    CREATE TABLE public.referenced_table (
+      id bigserial primary key not null
+    );
+
+    CREATE TABLE public.other_referenced_table (
+      id bigserial primary key not null
+    );
+
+    CREATE TABLE public.constrained_table (
+      id bigserial primary key not null,
+      referenced_table_id bigint not null,
+      other_referenced_table_id bigint not null,
+      CONSTRAINT fk_constrained_to_referenced FOREIGN KEY(referenced_table_id) REFERENCES referenced_table(id),
+      CONSTRAINT fk_constrained_to_other_referenced FOREIGN KEY(other_referenced_table_id)
+         REFERENCES other_referenced_table(id)
+    );
+    SQL
+  end
+
+  describe '#by_referenced_table_identifier' do
+    it 'throws an error when the identifier name is not fully qualified' do
+      expect { described_class.by_referenced_table_identifier('referenced_table') }.to raise_error(ArgumentError, /not fully qualified/)
+    end
+
+    it 'finds the foreign keys for the referenced table' do
+      expected = described_class.find_by!(name: 'fk_constrained_to_referenced')
+
+      expect(described_class.by_referenced_table_identifier('public.referenced_table')).to contain_exactly(expected)
+    end
+  end
+end