Coercing float to BigDecimal does not work

[tlconnor]

Given param definitions:

params do
  requires :amount, type: BigDecimal
end

and input JSON { "amount": "1.23" }, the endpoint will see params

{ amount: 0.123e1 } # 0.123e1 is a BigDecimal

If a Float is instead passed { "amount": 1.23 } the endpoint will then see params

{ amount: 1.23 } # 1.23 is a Float when it should be a BigDecimal

There is currently a test which covers this case, however it is testing the wrong thing. If I change the test to output the .class of the resulting value, you can see that it is a Float instead of a BigDecimal:

diff --git a/spec/grape/validations/validators/coerce_spec.rb b/spec/grape/validations/validators/coerce_spec.rb
index e77d839..4f5ed00 100644
--- a/spec/grape/validations/validators/coerce_spec.rb
+++ b/spec/grape/validations/validators/coerce_spec.rb
@@ -162,12 +162,12 @@ describe Grape::Validations::CoerceValidator do
             requires :bigdecimal, type: BigDecimal
           end
           subject.post '/bigdecimal' do
-            params[:bigdecimal]
+            params[:bigdecimal].class
           end
 
           post '/bigdecimal', { bigdecimal: 45.1 }.to_json, headers
           expect(last_response.status).to eq(201)
-          expect(last_response.body).to eq('45.1')
+          expect(last_response.body).to eq('BigDecimal')
         end

results in

Failures:

  1) Grape::Validations::CoerceValidator coerce coerces json BigDecimal
     Failure/Error: expect(last_response.body).to eq('BigDecimal')
     
       expected: "BigDecimal"
            got: "Float"
     
       (compared using ==)
     # ./spec/grape/validations/validators/coerce_spec.rb:170:in `block (5 levels) in <top (required)>'

The root cause of the problem can be found here https://github.com/ruby-grape/grape/blob/master/lib/grape/validations/validators/coerce.rb#L50 and was introduced in 9b352a88#diff-4b67e961836edb86baea810dc3156e24R48.

The problem is that the code uses == to determine if a param value has changed after coersion, however a Float and BigDecimal can == each other:

> 1.23 == BigDecimal('1.23')
=> true

The Float never gets replaced with the BigDecimal. A previous issue covers this same problem, but the fix did not work properly.
#1969

[dblock]

Looks like a bug to me, fix welcome, /cc: @dnesteryuk.

[dnesteryuk]

ok, it was fixed