Update

lib/eve_online/esi/universe_category.rb

@@ -9,16 +9,15 @@ class UniverseCategory < Base
 
       API_PATH = "/v1/universe/categories/%<category_id>s/"
 
-      attr_reader :id
+      attr_reader :category_id
 
       def initialize(options)
         super
 
-        @id = options.fetch(:id)
+        @category_id = options.fetch(:category_id)
       end
 
-      def_delegators :model, :as_json, :category_id, :name, :published,
-        :group_ids
+      def_delegators :model, :as_json, :id, :name, :published, :group_ids
 
       def model
         @model ||= Models::Category.new(response)
@@ -32,7 +31,7 @@ def additional_query_params
       end
 
       def path
-        format(API_PATH, category_id: id)
+        format(API_PATH, category_id: category_id)
       end
     end
   end

spec/eve_online/esi/universe_category_spec.rb

@@ -3,7 +3,7 @@
 require "spec_helper"
 
 describe EveOnline::ESI::UniverseCategory do
-  let(:options) { {id: 6} }
+  let(:options) { {category_id: 6} }
 
   subject { described_class.new(options) }
 
@@ -20,7 +20,7 @@
 
     its(:_write_timeout) { should eq(60) }
 
-    its(:id) { should eq(6) }
+    its(:category_id) { should eq(6) }
   end
 
   describe "#model" do
@@ -62,14 +62,14 @@
     specify { expect { subject.as_json }.not_to raise_error }
   end
 
-  describe "#category_id" do
+  describe "#id" do
     let(:model) { instance_double(EveOnline::ESI::Models::Category) }
 
     before { subject.instance_variable_set(:@model, model) }
 
-    before { expect(model).to receive(:category_id) }
+    before { expect(model).to receive(:id) }
 
-    specify { expect { subject.category_id }.not_to raise_error }
+    specify { expect { subject.id }.not_to raise_error }
   end
 
   describe "#name" do