rails uniqueness of two fields combination
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I have a model with field a, b. I want to make sure that the combination of a and b is unique in the table, so a=1, b=2 and a=2, b=1 would raise a conflict.

I have try `validates_uniqueness_of :a, :scope => :b`, but it only make sure that no two
a=1, b=2 row can exist, I also want to filter out the a=2, b=1.  Thanks