def self.up
  create_table :foo do |t|
    t.column "bar", :decimal
  end
end

There is an inherent precision of 10 and a scale of 0. What this means is that you can only do whole numbers. As it turns out precision stands for the total number of numerical charactors that field will store total and the scale is the number of numberical charecters the field will store after the decimal point. So for example a decimal with a precision of 8 and a scale of 2 will store numbers like this 123456.78 or 123.46(rounded up) This also means that the precision must always be greater than or equal to the scale.

Here is an example of how to set the precision and scale of a decimal type field manually.

def self.up
  create_table :foo do |t|
    t.column "bar", :decimal, :precision => 10, :scale => 6
  end
end

Hope this helps.
Best Luck