Support dynamic target field names.

It's conceivable that one would want the target field name to be
chosen dynamically using the contents of another field, so pass the
target option value through sprintf.

Author: magnusbaeck

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 {