Synvert snippet to keep foreign key name generated by `foreigner` gem for Rails 4.2 app.

foreigner.rb

# Synvert snippet to keep foreign key name generated by `foreigner` gem for Rails 4.2 app.
# Usage:
# Save to ~/.synvert/lib/rails/foreigner.rb
# $ cd RAILS_ROOT
# $ synvert --run rails/foreigner

require "active_support/inflector"
load "config/initializers/inflections.rb"

module Synvert::Core
  class Rewriter::InsertBeforeAction < Rewriter::Action
    def begin_pos
      @node.loc.expression.begin_pos - @node.indent
    end
    def end_pos
      begin_pos
    end

  private
    def indent(node)
      ' ' * node.indent
    end
  end
end

module Synvert::Core
  class Rewriter::Instance
    def insert_before(node)
      @actions << Rewriter::InsertBeforeAction.new(self, node + "\n")
    end
  end
end

Synvert::Rewriter.new 'rails', 'foreigner' do
  description <<-EOF
It converts rails migrations with foreigner to without foreigner.
  EOF

  within_file 'db/migrate/*.rb' do
    def has_name?(node)
      if node.arguments.last.type == :hash
        return true if node.arguments.last.has_key?(:name)
      end
    end

    def build_fk(from_table, to_table, hash_node = nil)
      if hash_node
        if hash_node.has_key?(:column)
          column = hash_node.hash_value(:column).to_value
        end
      end
      column ||= to_table.to_s.singularize + "_id"
      "#{from_table}_#{column}_fk"
    end

    def extract_hash(node)
      if node.arguments.last.type == :hash
        node.arguments.last
      end
    end

    %w(create_table change_table).each do |method_name|
      within_node type: 'block', caller: {type: 'send', message: method_name} do
        from_table = nil

        with_node type: 'send', message: method_name do
          from_table = node.arguments.first.to_value
        end

        lines_before_block = []
        lines_after_block = []

        with_node type: 'send', receiver: 't', message: :foreign_key do
          to_table = node.arguments[0].to_value
          hash = extract_hash(node)
          name = build_fk(from_table, to_table, hash)
          arguments = node.arguments.map(&:to_source)
          arguments.push(%(name: "#{name}"))
          lines_after_block << "add_foreign_key :#{from_table}, #{arguments.join(", ")}"
          remove
        end

        with_node type: 'send', receiver: 't', message: :remove_foreign_key do
          name = node.arguments[0].hash_value(:name).to_value
          lines_before_block << "remove_foreign_key :#{from_table}, name: #{name.dump}"
          remove
        end

        unless lines_before_block.empty?
          insert_before(lines_before_block.join("\n"))
        end

        unless lines_after_block.empty?
          insert_after(lines_after_block.join("\n"))
        end
      end
    end

    with_node type: 'send', message: :add_foreign_key do
      next if has_name?(node)
      hash = extract_hash(node)

      from_table = node.arguments[0].to_value
      to_table = node.arguments[1].to_value
      name = build_fk(from_table, to_table, hash)

      replace_with %(add_foreign_key {{arguments}}, name: #{name.dump})
    end

    with_node type: 'send', message: :remove_foreign_key do
      next if has_name?(node)
      hash = extract_hash(node)

      from_table = node.arguments[0].to_value
      to_table = node.arguments[1].to_value
      name = build_fk(from_table, to_table, hash)

      replace_with %(remove_foreign_key {{arguments.first}}, name: #{name.dump})
    end
  end
end