graph.rake

task graph: :environment do
  result = ActiveRecord::Base.connection.execute <<~SQL
    select source.relname as source, dest.relname as dest
    from pg_catalog.pg_constraint as fk
    join pg_catalog.pg_class as source on source.oid = fk.conrelid
    join pg_catalog.pg_class as dest   on   dest.oid = fk.confrelid
    where contype = 'f'::char
  SQL

  relations = result.map(&:symbolize_keys).reduce({}) do |h, source:, dest:|
    h.merge(source => [dest]) { |_, a, b| a + b }
  end

  puts <<~DOT
    digraph fks {
    \tlayout=neato;
    \tnodesep=1;
    \tnode [shape=box];
    #{relations.map { |table, refs| "\t#{table} -> {#{refs.join ' '}}" }.join ";\n"}
    }
  DOT
end