How to generate fixtures based on my development database?

The question is old, but as it still seems relevant: yes, there is an easy way to create fixtures from your development database:

class ActiveRecord::Base
  def dump_fixture
    fixture_file = "#{Rails.root}/test/fixtures/#{self.class.table_name}.yml"
    File.open(fixture_file, "a+") do |f|
      f.puts({ "#{self.class.table_name.singularize}_#{id}" => attributes }.
        to_yaml.sub!(/---\s?/, "\n"))
    end
  end
end

Place this in a file in config/initializers - now you can dump any ActiveRecord object in your Rails console and it will automatically be appended at the end of it's respective fixture file:

User.first.dump_fixture appends fixture data to test/fixtures/users.yml.

Building on @nikolasgd's answer I wrote a Rake Task, which may be useful for some:

# Use like "rake custom:create_test_fixtures[model]"
namespace :custom do
  desc 'Re-Creates Fixtures for Testing for a Model'

  task :create_test_fixtures, [:model] => [:environment] do |t, args|
    class ActiveRecord::Base
      def dump_fixture
        fixture_file = "#{Rails.root}/test/fixtures/#{self.class.table_name}.yml"
        File.open(fixture_file, "a+") do |f|
          f.puts({"#{self.class.table_name.singularize}_#{id}" => attributes}.
              to_yaml.sub!(/---\s?/, "\n"))
        end
      end
    end

    begin
      model = args[:model].constantize
      model.all.map(&:dump_fixture)
      puts "OK: Created Fixture for Model \"#{args[:model]}\"."
    rescue NameError
      puts "ERROR: Model \"#{args[:model]}\" not found."
    end

  end
end

In case you're creating a script to run under rails runner you can use the following approach:

File.open("#{Rails.root}/spec/fixtures/documents.yml", 'w') do |file|
  file.write Document.all.to_a.map(&:attributes).to_yaml
end

you can create as much blocks as you want, or if you want to go to the full database you can try:

models = defined?(AppicationRecord) ? ApplicationRecord.decendants : ActiveRecord::Base.descendants
models.each do |model|
  model_name = model.name.pluralize.underscore
  File.open("#{Rails.root}/spec/fixtures/#{model_name}.yml", 'w') do |file|
    file.write model.all.to_a.map(&:attributes).to_yaml
  end
end

if you do not want the timestamps you can change the code to: model.all.to_a.map { |m| m.attributes.except('created_at', 'updated_at')}.to_yaml