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Apply scale before precision when coercing floats to decimal

Since precision is always larger than scale, it can actually change
rounding behavior. Given a precision of 5 and a scale of 3, when you
apply the precision of 5 to `1.25047`, the result is `1.2505`, which
when the scale is applied would be `1.251` instead of the expected
`1.250`.

This issue appears to only occur with floats, as scale doesn't apply to
other numeric types, and the bigdecimal constructor actually ignores
precision entirely when working with strings. There's no way we could
handle this for the "unknown object which responds to `to_d`" case, as
we can't assume an interface for applying the scale.

Fixes #24235
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sgrif committed Mar 24, 2016
1 parent a12ad8a commit c7d3bd48dff0a509f5c21ec8864cb4f774d604e6
Showing with 17 additions and 2 deletions.
  1. +10 −2 activemodel/lib/active_model/type/decimal.rb
  2. +7 −0 activemodel/test/cases/type/decimal_test.rb
@@ -29,12 +29,12 @@ def cast_value(value)
end
end
scale ? casted_value.round(scale) : casted_value
apply_scale(casted_value)
end
def convert_float_to_big_decimal(value)
if precision
BigDecimal(value, float_precision)
BigDecimal(apply_scale(value), float_precision)
else
value.to_d
end
@@ -47,6 +47,14 @@ def float_precision
precision.to_i
end
end
def apply_scale(value)
if scale
value.round(scale)
else
value
end
end
end
end
end
@@ -52,6 +52,13 @@ def test_changed?
assert_not type.changed?(5.0, 5.0, '5.0')
assert_not type.changed?(-5.0, -5.0, '-5.0')
end
def test_scale_is_applied_before_precision_to_prevent_rounding_errors
type = Decimal.new(precision: 5, scale: 3)
assert_equal BigDecimal("1.250"), type.cast(1.250473853637869)
assert_equal BigDecimal("1.250"), type.cast("1.250473853637869")
end
end
end
end

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