module ForeignKeyChecker
  module Utils
    class Result
      attr_reader :from_table, :to_table, :from_column, :to_column
      def initialize(args)
        args.each { |k, v| instance_variable_set("@#{k}", v) }
      end
    end
    class UnsupportedConnectionAdapter < StandardError; end
    def self.get_foreign_keys(model = ActiveRecord::Base)
      adapter = model.connection_config[:adapter]
      raise(UnsupportedConnectionAdapter, adapter) unless %w[postgresql mysql2].include?(adapter)

      connection = model.connection
      send("get_#{adapter}_foreign_keys", connection)
    end

    def self.get_foreign_keys_hash(model = ActiveRecord::Base)
      get_foreign_keys(model).to_a.each_with_object({}) do |datum, obj|
        obj[datum.to_table] ||= []
        obj[datum.to_table].push(datum)
      end
    end

    def self.get_mysql2_foreign_keys(connection)
      res = connection.select_all <<-SQL
        SELECT
          fks.TABLE_NAME AS from_table,
          fks.COLUMN_NAME AS from_column,
          fks.REFERENCED_TABLE_NAME AS to_table,
          fks.REFERENCED_COLUMN_NAME AS to_column
        FROM information_schema.KEY_COLUMN_USAGE AS fks
        INNER JOIN information_schema.REFERENTIAL_CONSTRAINTS rules ON rules.CONSTRAINT_NAME = fks.CONSTRAINT_NAME
        WHERE
          fks.CONSTRAINT_SCHEMA = DATABASE()
          AND rules.CONSTRAINT_SCHEMA = DATABASE();
      SQL
      res.to_a.map{|i| Result.new(i) }
    end

    def self.get_postgresql_foreign_keys(connection)
      res = connection.select_all <<-SQL
        SELECT
          tc.table_name AS from_table,
          kcu.column_name AS from_column,
          ccu.table_name AS to_table,
          ccu.column_name AS to_column
        FROM
          information_schema.table_constraints AS tc
          JOIN information_schema.key_column_usage AS kcu
            ON tc.constraint_name = kcu.constraint_name
            AND tc.table_schema = kcu.table_schema
          JOIN information_schema.constraint_column_usage AS ccu
            ON ccu.constraint_name = tc.constraint_name
            AND ccu.table_schema = tc.table_schema
        WHERE tc.constraint_type = 'FOREIGN KEY';
      SQL
      res.to_a.map{ |i| Result.new(i) }
    end
  end
end