Add Logger#[] and Logger#[]= alias methods to get and set fields

lib/rackstash/logger.rb

@@ -83,6 +83,28 @@ module Rackstash
       msg
     end
 
+    # Retrieve a stored value from a given `key` in the current Buffer's fields.
+    # This is strictly equivalent to calling `logger.fields[key]`.
+    #
+    # @param (see Fields::Hash#[])
+    # @return (see Fields::Hash#[])
+    def [](key)
+      buffer.fields[key]
+    end
+
+    # Set the value of a key in the current Buffer's fields to the supplied
+    # value. You can set nested hashes and arrays here. The hash keys will be
+    # normalized as strings.
+    # This is strictly equivalent to calling `logger.fields[key] = value`.
+    #
+    # @param (see Fields::Hash#[]=)
+    # @raise [ArgumentError] if you attempt to set one of the forbidden fields,
+    #   namely any of {Buffer::FORBIDDEN_FIELDS}
+    # @return (see Fields::Hash#[]=)
+    def []=(key, value)
+      buffer.fields[key] = value
+    end
+
     # Set the base log level as either one of the {SEVERITIES} or a
     # String/Symbol describing the level. When logging a message, it will only
     # be added if its log level is at or above the base level defined here

spec/rackstash/logger_spec.rb

@@ -38,6 +38,32 @@ describe Rackstash::Logger do
     end
   end
 
+  describe 'subscript accessors' do
+    it 'gets a fields from the current Buffer' do
+      logger['key'] = 'value'
+      expect(logger['key']).to eql 'value'
+    end
+
+    it 'normalizes keys when setting values' do
+      logger[:foo] = 'foo value'
+      expect(logger['foo']).to eql 'foo value'
+
+      logger[42] = '42 value'
+      expect(logger['42']).to eql '42 value'
+    end
+
+    it 'returns nil if a value was not set' do
+      expect(logger['missing']).to be_nil
+    end
+
+    it 'can\'t set forbidden values' do
+      expect { logger['message'] = 'nope' }.to raise_error ArgumentError
+      expect { logger['tags'] = 'nope' }.to raise_error ArgumentError
+      expect { logger['@timestamp'] = 'nope' }.to raise_error ArgumentError
+      expect { logger['@version'] = 'nope' }.to raise_error ArgumentError
+    end
+  end
+
   describe '#close' do
     it 'forwards to the sink' do
       expect(logger.sink).to receive(:close)