Validate using boolean values

The Problem: how to do the validation. I have not written it

Given two tables: sizes and prices;
with a relation: has_many ,belongs_to

a Price.unit_price for a given Size.meassure must be
marked ‘true’ as the standard unit_price for that meassure.

There will be many other unit_prices for a given meassure, those must be
marked ‘false’

table sizes
meassure, :string (ie.small, medium, big)

Model Size
has_many :prices

table prices
price_name :string
unit_price :integer
standard :boolean
size_id :integer

Model Price
belongs_to :size

I have tried both of this validation options in the Price model,
first one, then the other and none works for my stated needs.

validates_uniqueness_of :standard, :scope => :size_id,
:message => " : There is a standard price
for this meassure"

(this one fails because it just allows ONE ‘true’ in a :standard
field in the entire
database for ANY value of size_id; so once there is a ‘true’ in the
:standard field of the whole database, it will not allow any more;
so tha does not fit me…)

def validate
errors.add(:standard, " : There is a standard price for this
meassure") if
size_id and standard == ‘true’

(this one fails because it allows to have many ‘true’ values in
the :standard
field of the database)

This is what i need to get:

I am using MySQL, notice that MySQL writes ‘true’ = 1 and ‘false’ = 0;
also see that for a given unit_price for a meassure
there can only be one standard = ‘true’=1 value.

                   sizes  (in the Size model

validates_uniqueness_of :meassure)

id meassure

1 Small
2 Medium
3 Big


id price_name unit_price standard size_id

1 normal 1250 1 1
2 sale 500 0 1
3 summer 900 0 1
4 wholesale 1000 0 2
5 normal 1900 1 2
6 winter 1500 0 2
7 normal 3000 1 3
8 thanksgiving 2500 0 3
9 sale 1000 0 3

Please help me write up the correct way to validate this problem

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