Support dynamic target field names

lib/logstash/filters/json.rb

@@ -52,6 +52,12 @@ class LogStash::Filters::Json < LogStash::Filters::Base
   # JSON in the value of the `source` field will be expanded into a
   # data structure in the `target` field.
   #
+  # The string in the `target` option may contain dynamic sprintf-style
+  # field references, i.e. the contents of one field can be used to choose
+  # the name of the target field. Dynamic field references can only refer
+  # to already existing fields, so the JSON string being parsed can't
+  # contain the name of the target field.
+  #
   # NOTE: if the `target` field already exists, it will be overwritten!
   config :target, :validate => :string
 
@@ -83,7 +89,7 @@ def filter(event)
     end
 
     if @target
-      event.set(@target, parsed)
+      event.set(event.sprintf(@target), parsed)
     else
       unless parsed.is_a?(Hash)
         @tag_on_failure.each{|tag| event.tag(tag)}

spec/filters/json_spec.rb

@@ -41,6 +41,23 @@
     end
   end
 
+  describe "parse message into a dynamically named target field" do
+    config <<-CONFIG
+      filter {
+        json {
+          # Parse message as JSON, store the results in the field named
+          # by the contents of the 'target' field'
+          source => "message"
+          target => "%{target}"
+        }
+      }
+    CONFIG
+
+    sample({"target" => "data", "message" => '{ "hello": "world" }'}) do
+      insist { subject.get("[data][hello]") } == "world"
+    end
+  end
+
   describe "tag invalid json" do
     config <<-CONFIG
       filter {