Version information:
MYSQL DB version: 5.5.38-0ubuntu0.14.04.1
Rails version: 4.1.0
Ruby version: 2.1.0p0 (2013-12-25 revision 44422) [x86_64-linux]

I created a table with the following migration:
create_table "my_table", force: true do |t|
t.decimal "my_column", default: 0.0
end

When attempting to assign values to the column, the values would be treated as integers. In the MYSQL console, the column was reported as DECIMAL(10,0) with a default of 0. In the rails console, the column was reported as type :integer and acted as one.

No idea what caused this, but setting precision: 10, scale: 2 caused rails to treat the column as a :decimal again.