Preserve existing field mappings for encoder fields

lib/rackstash/encoder/helper/fields_map.rb

@@ -18,19 +18,29 @@ module Rackstash
         private
 
         def set_fields_mapping(fields, default = {})
-          @fields_map = default.dup
-          Hash(fields).each_pair do |key, value|
-            @fields_map[key.to_sym] = utf8_encode(value)
+          @fields_map ||= {}
+          default.each do |key, value|
+            if fields.key?(key)
+              @fields_map[key] = utf8_encode(fields[key])
+            else
+              # Preserve existing mappings which might have been set by a
+              # previous call to {#set_fields_mapping}
+              @fields_map[key] ||= utf8_encode(value)
+            end
           end
         end
 
         def extract_field(name, event)
-          field_name = @fields_map[name]
+          field_name = field(name)
 
           field = event.delete(field_name) if field_name
           field = yield(field_name) if field.nil? && block_given?
           field
         end
+
+        def field(name)
+          @fields_map[name]
+        end
       end
     end
   end

spec/rackstash/encoder/helper/fields_map_spec.rb

@@ -26,6 +26,44 @@ RSpec.describe Rackstash::Encoder::Helper::FieldsMap do
     }
   }
 
+  describe '#set_fields_mapping' do
+    it 'sets a default mapping' do
+      helper.set_fields_mapping({}, { field: 'default' })
+      expect(helper.field(:field)).to eql 'default'
+    end
+
+    it 'sets default fields as strings' do
+      helper.set_fields_mapping({}, { number: 123 })
+      expect(helper.field(:number)).to eql '123'
+    end
+
+    it 'sets fields as strings' do
+      helper.set_fields_mapping({ number: 42 }, { number: 'something' })
+      expect(helper.field(:number)).to eql '42'
+    end
+
+    it 'overwrites fields on subsequent calls' do
+      helper.set_fields_mapping({ field: 'overwritten' }, { field: 'default' })
+      expect(helper.field(:field)).to eql 'overwritten'
+
+      helper.set_fields_mapping({ field: 'again' }, { field: 'other_default' })
+      expect(helper.field(:field)).to eql 'again'
+    end
+
+    it 'keeps existing default fields on subsequent calls' do
+      helper.set_fields_mapping({}, { field: 'foo' })
+      expect(helper.field(:field)).to eql 'foo'
+
+      helper.set_fields_mapping({}, { field: 'bar' })
+      expect(helper.field(:field)).to eql 'foo'
+    end
+
+    it 'ignores fields not defined as a default field' do
+      helper.set_fields_mapping({invalid: 'invalid' }, { known: 'known' })
+      expect(helper.field(:invalid)).to be_nil
+    end
+  end
+
   describe '#extract_field' do
     context 'with defaults' do
       let(:defaults) { { default: 'foo' } }