schema2dot.rb

=begin
  Read the db/schema.rb file and churn out a .DOT file containing
  tables and linkages for *all* tables, not just the ones that
  have ActiveRecord models. Guess that columns ending in _id are
  links to other tables; assume all relations are one-to-many.

  BACKGROUND
  I'm working with a schema that is shared by a Rails app and a C++ app.
  Some tables are only accessed by the C++ code and have no need to be
  represented in the Rails models. A side-effect of this is that a tool
  like the Railroad gem only shows half the schema. So I wrote this code
  to create a schema diagram similar to what Railroad creates.
  It requires the graphviz "dot" appliation to do the final rendering from
  a .DOT file to a .JPG file.

  USAGE (from your Rails root):
    schema2dot.rb > full_models.dot
    dot -Tjpg full_models.dot > full_models.jpg

  CAVEATS:
  1) we treat everything as a one-to-many relationship, so if you have
  a "has one" relationship we will fail to show an empty circle on the
  "one" side, which would correctly (if I understand UML) denote that
  it was a has-one instead of a has-many relationship.
  2) Requires that the entire schema is represented in the db/schema.rb
  file (i.e. that Rails migrations created all the tables). In theory
  the C++ code could separately create the tables that it "owns" but
  that would likely be unworkable in the long term.

  TODOs that I'll probably never do:
  - use the Rails-standard singularize() method instead of just 
    dropping the final 's' of the table name.

=end

def convert_column_to_link column_name
  cn = column_name.dup
  if cn =~ /^(.*)_id$/
    return $1
  else
    return nil
  end
end

def interesting_table table_name
  true #your schema-specific filter goes here
end

DOT_OUTPUT = 'digraph models_diagram {
  graph[overlap=false, splines=true]
#{linked_tables_str}
#{linkages_str}
}'

TABLE_LINE = '  \"#{table_name}\" [shape=Mrecord, label=\"{#{table_name}#{table_columns}}\"]'
LINK_LINE =  '  \"#{has_many_table_name}\" -> \"#{table_name}\" [arrowtail=crow, arrowhead=dot, dir=both]'

TABLE_REGEXP = /\screate_table\s+"(\w+)s"(.*?)\s+end/m
ROW_REGEXP   = /t\.column\s+"(\w+)",\s+\:(\w+)/m

schema = IO.read('db/schema.rb')
tables = {}
linkages = {}
linked_tables = {}
schema.scan(TABLE_REGEXP).each do |table_name, table_contents|
  if interesting_table(table_name)
    tables[table_name] ||= []
    linkages[table_name] ||= []
    columns = table_contents.scan(ROW_REGEXP).each do |column_name, column_type|
      tables[table_name] << [column_name, column_type]
      link_name = convert_column_to_link(column_name)
      if link_name
        linkages[table_name] << link_name
        linked_tables[table_name] = true
        linked_tables[link_name] = true
      end
    end
  end
end
linked_tables_array = []
linkages_array = []
linked_tables.keys.sort.each do |table_name|
  if tables[table_name]
    table_columns = tables[table_name].collect { |ar| "#{ar.first} :#{ar.last}\\l" }.join
    table_columns = '|'+table_columns if table_columns
    linked_tables_array << eval('"'+TABLE_LINE+'"')
    linkages[table_name].uniq.sort.each do |has_many_table_name|
      if tables.keys.include? has_many_table_name
        linkages_array << eval('"'+LINK_LINE+'"')
      end
    end
  end
end

linked_tables_str = linked_tables_array.join("\n")
linkages_str = linkages_array.join("\n")

result = eval('"'+DOT_OUTPUT+'"')
puts result