bq_schema.rb

require 'json'

class Schema
  attr_reader :items

  def initialize(&block)
    @items = []
    instance_eval &block
  end

  def field(name, type, options = {})
    @items << {name: name, type: type.upcase}.merge(options)
  end

  %w(string integer float boolean).each do |type|
    define_method(type) do |name|
      field name, type
    end
  end

  def fields(name, &block)
    field name, :record, fields: Schema.new(&block).items
  end

  def to_json
    JSON.pretty_generate @items
  end
end

schema = Schema.new do
  integer :id
  integer :created_at
  boolean :favorited
  fields :user do
    integer :id
    string :screen_name
    string :name
  end
end

puts schema.to_json