Allow nil in length validator

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 #### Features
 
+* [#2464](https://github.com/ruby-grape/grape/pull/2464): Allow `nil` in `length` validator - [@OuYangJinTing](https://github.com/OuYangJinTing).
 * Your contribution here.
 
 #### Fixes

README.md

@@ -1713,8 +1713,8 @@ end
 #### `length`
 
 Parameters with types that support `#length` method can be restricted to have a specific length with the `:length` option.
-
-The validator accepts `:min` or `:max` or both options to validate that the value of the parameter is within the given limits.
+In addition, if the received parameter value is `nil`, the length validation will not be triggered.
+If you want to reject `nil`, you can use the `allow_blank: false` option (This will also reject empty strings).
 
 ```ruby
 params do

lib/grape/validations/validators/length_validator.rb

@@ -18,6 +18,8 @@ def initialize(attrs, options, required, scope, **opts)
         def validate_param!(attr_name, params)
           param = params[attr_name]
 
+          return if param.nil?
+
           raise ArgumentError, "parameter #{param} does not support #length" unless param.respond_to?(:length)
 
           return unless (!@min.nil? && param.length < @min) || (!@max.nil? && param.length > @max)

spec/grape/validations/validators/length_spec.rb

@@ -81,6 +81,18 @@
       post 'zero_max' do
       end
 
+      params do
+        requires :list, type: [Integer], length: { min: 2 }
+      end
+      post 'nil_param' do
+      end
+
+      params do
+        requires :list, type: [Integer], length: { min: 2 }, allow_blank: false
+      end
+      post 'length_with_disallow_blank' do
+      end
+
       params do
         requires :list, type: [Integer], length: { min: 2, message: 'not match' }
       end
@@ -187,6 +199,42 @@
     end
   end
 
+  describe '/nil_param' do
+    context 'does not raise an error' do
+      it do
+        expect do
+          post '/nil_param', list: nil
+        end.not_to raise_error
+      end
+    end
+  end
+
+  describe '/length_with_disallow_blank' do
+    context 'when length is within limits' do
+      it do
+        post '/length_with_disallow_blank', list: [1, 2]
+        expect(last_response.status).to eq(201)
+        expect(last_response.body).to eq('')
+      end
+    end
+
+    context 'when a nil value is passed' do
+      it do
+        post '/length_with_disallow_blank', list: nil
+        expect(last_response.status).to eq(400)
+        expect(last_response.body).to eq('list is empty')
+      end
+    end
+
+    context 'when a empty string is passed' do
+      it do
+        post '/length_with_disallow_blank', list: ''
+        expect(last_response.status).to eq(400)
+        expect(last_response.body).to eq('list is expected to have length greater than or equal to 2, list is empty')
+      end
+    end
+  end
+
   describe '/type_is_not_array' do
     context 'raises an error' do
       it do