Optionally prefix all message lines with extraced fields in Message encoder

This can be used to achieve similar results as
ActiveSupport::TaggedLogger. With our implementation, the "tags" are
prepended to each line (instead of each message) and we add the same
tags to all message lines.

If required, you can still use ActiveSupport::TaggedLogger with
Rackstash to add tags at the time of adding the log message.

lib/rackstash/encoders/message.rb

@@ -9,21 +9,59 @@ require 'rackstash/encoders/helpers/message'
 
 module Rackstash
   module Encoders
-    # The Message encoder only returns the message of log event. All other
-    # fields and tags are ignored.
+    # The Message encoder only returns the formatted message of log event. All
+    # other fields and tags are ignored. This encoder is useful in environments
+    # where the added fields are not required, mostly during development where
+    # (debug) logs are directly consumed by humans.
     #
-    # This encoder is useful in environments where the added fields are not
-    # required, mostly during development where debug logs are directly consumed
-    # by humans
+    # If any `prefix_fields` are given in the initializer, we gather their
+    # values from the event and prefix them to each line in the message if the
+    # current event contains a value at the given field names.
+    #
+    #     encoder = Rackstash::Encoders::Message.new(tagged: ['tags', 'remote_ip'])
+    #
+    #     event = {
+    #       'remote_ip' => '127.0.0.1',
+    #       'tags' => ['foo', 123],
+    #       'message' => ["Hello\n", "World\n"],
+    #       'key' => 'value'
+    #     }
+    #     encoder.encode(event)
+    #     # Logs "[foo,123] [127.0.0.1] Hello\n[foo,123] [127.0.0.1] World\n"
     class Message
       include Rackstash::Encoders::Helpers::Message
 
+      # @param [Array<#to_s>] An array of field names whose values are added in
+      #   front of each message line on encode
+      def initialize(tagged: [])
+        @tagged_fields = Array(tagged).map(&:to_s)
+      end
+
+      # Return the formatted message of the given event.
+      #
       # @param event [Hash] a log event as produced by the {Flow}
-      # @return [String] the `"message"` field of the event
+      # @return [String] the formatted message of the event
       def encode(event)
         normalize_message(event)
+        message = event[FIELD_MESSAGE]
 
-        event[FIELD_MESSAGE]
+        tags = @tagged_fields.map { |key| format_tag event[key] }.compact.join
+        message = message.gsub(/^/) { tags } unless tags.empty?
+
+        message
+      end
+
+      private
+
+      def format_tag(value)
+        case value
+        when nil
+          nil
+        when Array
+          "[#{value.map(&:to_s).join(',')}] "
+        else
+          "[#{value}] "
+        end
       end
     end
   end

spec/rackstash/encoders/message_spec.rb

@@ -10,12 +10,39 @@ require 'spec_helper'
 require 'rackstash/encoders/message'
 
 describe Rackstash::Encoders::Message do
-  let(:encoder) { described_class.new }
+  let(:tagged) { [] }
+  let(:encoder) { described_class.new(tagged: tagged) }
 
   describe '#encode' do
     it 'gets the message from the event hash' do
       event = { 'hello' => 'world', 'message' => ["\n\t \nline1\n", "line2\n  \n\t\n"] }
       expect(encoder.encode(event)).to eql "\n\t \nline1\nline2\n  \n\t\n"
     end
+
+    context 'with prefix_fields' do
+      let(:tagged) { [:sym, 'field', 'tags'] }
+
+      it 'adds fields to all lines' do
+        event = { 'message' => ["line1\t\n", "line2\nline3\n\t\n"], 'field' => 'BXC' }
+        expect(encoder.encode(event))
+          .to eql "[BXC] line1\t\n[BXC] line2\n[BXC] line3\n[BXC] \t\n"
+      end
+
+      it 'uses stringified fields' do
+        event = { 'message' => ["line1\n", "line2\nline3\n"], 'sym' => 'SYM', 'field' => 123 }
+        expect(encoder.encode(event))
+          .to eql "[SYM] [123] line1\n[SYM] [123] line2\n[SYM] [123] line3\n"
+      end
+
+      it 'formats arrays' do
+        event = { 'message' => ["line1\n", "line2\n"], 'tags' => ['foo', 'bar'] }
+        expect(encoder.encode(event)).to eql "[foo,bar] line1\n[foo,bar] line2\n"
+      end
+
+      it 'ignores missing fields' do
+        event = { 'message' => ["line1\n", "line2\n"] }
+        expect(encoder.encode(event)).to eql "line1\nline2\n"
+      end
+    end
   end
 end