MSch/formatted_big_decimals.rb

## formatted_big_decimals.rb
require 'rubygems'
require 'sinatra'
require 'dm-core'

DataMapper::Logger.new($stdout, :debug)
DataMapper.setup(:default, 'sqlite3::memory:')

# Don't use this in production code! Extending lots of objects during runtime
# will have a negative impact on performance/memory because ruby clears out
# the method cache.
#
# A better approach to implement this is to write a custom DataMapper::Type that
# delegates all methods to the underlying BigDecimal and just overwrites #to_s
# Why on earth haven't I thought about that before! Thx dkubb :P
#
#
# When this module is activated, it will round all BigDecimal property values
# to their specified :scale option when BigDecimal#to_s is called. This is
# achieved by overwriting all property readers for properties of type BigDecimal
# and then implementing the #to_s method with "round to scale" semantics on the
# metaclass of the BigDecimal instance to be returned. This way we can make sure
# that any occurrences of BigDecimals in web application views will always be
# displayed properly according to their specified :scale option. It can be argued
# that this functionality would be best kept in helpers, but the situation at hand
# didn't allow that since there are simply too many occurrences and we needed a
# quick fix.
#
# For the lazy, the module can be activated once and for all by e.g putting this into
# the merb after_app_loads block or similar places in other webframeworks.
#
#   DataMapper::Model.descendants.each do |model|
#     model.extend BigDecimalFormatting
#     model.format_big_decimals
#   end
#
# Of course, it's cleaner to just activate it by extending it into relevant models and
# calling the #format_big_decimals method explicitly *after* declaring all properties
#
# In the process of coming up with this solution I was surprised to see that it's
# neither possible to redefine BigDecimal#to_s by implementing it on a BigDecimal metaclass,
# nor by including a module that overwrites #to_s in BigDecimal. I have no clue about the
# latter, but the former issue always raised a Type error, saying
#
#   TypeError: can't define singleton method "to_s" for BigDecimal
#
# However, it seems the method is still attached to the metaclass, so either just
# swallowing the TypeError, or overwriting #singletion_method_added seem to do the trick
# This was definitely surprising behavior!
#
class BigDecimal
  def singleton_method_added(id)
  end
end

module BigDecimalFormatting

  def format_big_decimals
    properties.reject { |prop| prop.type != BigDecimal }.each do |prop|
      class_eval <<-RUBY, __FILE__, __LINE__
        def #{prop.name}
          result = attribute_get(#{prop.name.inspect})
          if result
            def result.to_s(s = 'F')
              round(#{prop.scale}).to_f.to_s
            end
          end
          result
        end
      RUBY
    end
  end

end

class Quote

  include DataMapper::Resource
  extend BigDecimalFormatting

  property :id,          Serial
  property :price,       BigDecimal, :precision => 20, :scale => 2
  property :click_price, BigDecimal, :precision =>  8, :scale => 5

  format_big_decimals

end

DataMapper.auto_migrate!


# ----------------------------------------------------------------
#                         DEMO APPLICATION
# ----------------------------------------------------------------

get '/quotes' do
  erb :index
end

get '/quotes/new' do
  @quote = Quote.new
  erb :new
end

post '/quotes' do
  @quote = Quote.create(params[:quote])
  redirect '/quotes'
end

get '/quotes/:id' do
  @quote = Quote.get(params[:id])
  erb :show
end

get '/quotes/:id/edit' do
  @quote = Quote.get(params[:id])
  erb :edit
end

put '/quotes/:id' do
  @quote = Quote.get(params[:id])
  halt 404 unless @quote
  @quote.update(params[:quote])
  redirect '/quotes'
end

delete '/quotes/:id' do
  @quote = Quote.get(params[:id])
  #halt 404 unless @quote
  @quote.destroy
  redirect '/quotes'
end

__END__

@@ layout
<!DOCTYPE html>
<html>
  <head><title>BigDecimal Standalone Demo</title></head>
  <body>
    <%= yield %>
  </body>
</html>

@@ index
<h3>Listing Quotes</h3>
<table>
  <tr>
    <th>Id</th>
    <th>Price</th>
    <th>ClickPrice</th>
  </tr>
<% Quote.all.each do |quote| %>
  <tr>
    <td><%= quote.id %></td>
    <td><%= quote.price %></td>
    <td><%= quote.click_price %></td>
    <td><a href="/quotes/<%= quote.id%>/edit">Edit</a></td>
    <td>
      <form action="/quotes/<%= quote.id %>" method="post">
        <input type="hidden" name="_method" value="delete" />
        <button type="submit">Delete</button>
      </form>
    </td>
  </tr>
<% end %>
</table>
<a href="/quotes/new">New Quote</a>

@@ new
<h3>New Quote</h3>
<form action="/quotes" method="post">
  <label for="quote[price]">Price</label>
  <input id="quote_price" name="quote[price]" value="<%= @quote.price %>">
  <label for="quote[click_price]">ClickPrice</label>
  <input id="quote_click_price" name="quote[click_price]" value="<%= @quote.click_price %>">
  <input type="submit">
</form>

@@ edit
<h3>Edit Quote</h3>
<form action="/quotes/<%= @quote.id %>" method="post">
  <input type="hidden" name="_method" value="put" />
  <label for="quote[price]">Price</label>
  <input id="quote_price" name="quote[price]" value="<%= @quote.price %>">
  <label for="quote[click_price]">ClickPrice</label>
  <input id="quote_click_price" name="quote[click_price]" value="<%= @quote.click_price %>">
  <input type="submit" />
</form>

@@ show
<h3>Show Quote</h3>
<label for="quote[price]">Price</label>
<p><%= @quote.price %></p>
<label for="quote[click_price]">ClickPrice</label>
<p><%= @quote.click_price %></p>
	require 'rubygems'
	require 'sinatra'
	require 'dm-core'

	DataMapper::Logger.new($stdout, :debug)
	DataMapper.setup(:default, 'sqlite3::memory:')

	# Don't use this in production code! Extending lots of objects during runtime
	# will have a negative impact on performance/memory because ruby clears out
	# the method cache.
	#
	# A better approach to implement this is to write a custom DataMapper::Type that
	# delegates all methods to the underlying BigDecimal and just overwrites #to_s
	# Why on earth haven't I thought about that before! Thx dkubb :P
	#
	#
	# When this module is activated, it will round all BigDecimal property values
	# to their specified :scale option when BigDecimal#to_s is called. This is
	# achieved by overwriting all property readers for properties of type BigDecimal
	# and then implementing the #to_s method with "round to scale" semantics on the
	# metaclass of the BigDecimal instance to be returned. This way we can make sure
	# that any occurrences of BigDecimals in web application views will always be
	# displayed properly according to their specified :scale option. It can be argued
	# that this functionality would be best kept in helpers, but the situation at hand
	# didn't allow that since there are simply too many occurrences and we needed a
	# quick fix.
	#
	# For the lazy, the module can be activated once and for all by e.g putting this into
	# the merb after_app_loads block or similar places in other webframeworks.
	#
	# DataMapper::Model.descendants.each do \|model\|
	# model.extend BigDecimalFormatting
	# model.format_big_decimals
	# end
	#
	# Of course, it's cleaner to just activate it by extending it into relevant models and
	# calling the #format_big_decimals method explicitly after declaring all properties
	#
	# In the process of coming up with this solution I was surprised to see that it's
	# neither possible to redefine BigDecimal#to_s by implementing it on a BigDecimal metaclass,
	# nor by including a module that overwrites #to_s in BigDecimal. I have no clue about the
	# latter, but the former issue always raised a Type error, saying
	#
	# TypeError: can't define singleton method "to_s" for BigDecimal
	#
	# However, it seems the method is still attached to the metaclass, so either just
	# swallowing the TypeError, or overwriting #singletion_method_added seem to do the trick
	# This was definitely surprising behavior!
	#
	class BigDecimal
	def singleton_method_added(id)
	end
	end

	module BigDecimalFormatting

	def format_big_decimals
	properties.reject { \|prop\| prop.type != BigDecimal }.each do \|prop\|
	class_eval <<-RUBY, __FILE__, __LINE__
	def #{prop.name}
	result = attribute_get(#{prop.name.inspect})
	if result
	def result.to_s(s = 'F')
	round(#{prop.scale}).to_f.to_s
	end
	end
	result
	end
	RUBY
	end
	end

	end

	class Quote

	include DataMapper::Resource
	extend BigDecimalFormatting

	property :id, Serial
	property :price, BigDecimal, :precision => 20, :scale => 2
	property :click_price, BigDecimal, :precision => 8, :scale => 5

	format_big_decimals

	end

	DataMapper.auto_migrate!


	# ----------------------------------------------------------------
	# DEMO APPLICATION
	# ----------------------------------------------------------------

	get '/quotes' do
	erb :index
	end

	get '/quotes/new' do
	@quote = Quote.new
	erb :new
	end

	post '/quotes' do
	@quote = Quote.create(params[:quote])
	redirect '/quotes'
	end

	get '/quotes/:id' do
	@quote = Quote.get(params[:id])
	erb :show
	end

	get '/quotes/:id/edit' do
	@quote = Quote.get(params[:id])
	erb :edit
	end

	put '/quotes/:id' do
	@quote = Quote.get(params[:id])
	halt 404 unless @quote
	@quote.update(params[:quote])
	redirect '/quotes'
	end

	delete '/quotes/:id' do
	@quote = Quote.get(params[:id])
	#halt 404 unless @quote
	@quote.destroy
	redirect '/quotes'
	end

	__END__

	@@ layout
	<!DOCTYPE html>
	<html>
	<head><title>BigDecimal Standalone Demo</title></head>
	<body>
	<%= yield %>
	</body>
	</html>

	@@ index
	<h3>Listing Quotes</h3>
	<table>
	<tr>
	<th>Id</th>
	<th>Price</th>
	<th>ClickPrice</th>
	</tr>
	<% Quote.all.each do \|quote\| %>
	<tr>
	<td><%= quote.id %></td>
	<td><%= quote.price %></td>
	<td><%= quote.click_price %></td>
	<td><a href="/quotes/<%= quote.id%>/edit">Edit</a></td>
	<td>
	<form action="/quotes/<%= quote.id %>" method="post">
	<input type="hidden" name="_method" value="delete" />
	<button type="submit">Delete</button>
	</form>
	</td>
	</tr>
	<% end %>
	</table>
	<a href="/quotes/new">New Quote</a>

	@@ new
	<h3>New Quote</h3>
	<form action="/quotes" method="post">
	<label for="quote[price]">Price</label>
	<input id="quote_price" name="quote[price]" value="<%= @quote.price %>">
	<label for="quote[click_price]">ClickPrice</label>
	<input id="quote_click_price" name="quote[click_price]" value="<%= @quote.click_price %>">
	<input type="submit">
	</form>

	@@ edit
	<h3>Edit Quote</h3>
	<form action="/quotes/<%= @quote.id %>" method="post">
	<input type="hidden" name="_method" value="put" />
	<label for="quote[price]">Price</label>
	<input id="quote_price" name="quote[price]" value="<%= @quote.price %>">
	<label for="quote[click_price]">ClickPrice</label>
	<input id="quote_click_price" name="quote[click_price]" value="<%= @quote.click_price %>">
	<input type="submit" />
	</form>

	@@ show
	<h3>Show Quote</h3>
	<label for="quote[price]">Price</label>
	<p><%= @quote.price %></p>
	<label for="quote[click_price]">ClickPrice</label>
	<p><%= @quote.click_price %></p>