From 4fb72d8877c2743e41215ffdc85616e3a7f40c15 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Magnus=20B=C3=A4ck?= <magnus@noun.se>
Date: Sun, 22 May 2016 22:03:47 +0200
Subject: [PATCH] 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.
---
 lib/logstash/filters/json.rb |  8 +++++++-
 spec/filters/json_spec.rb    | 17 +++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/lib/logstash/filters/json.rb b/lib/logstash/filters/json.rb
index b542431..010e5ec 100644
--- a/lib/logstash/filters/json.rb
+++ b/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)}
diff --git a/spec/filters/json_spec.rb b/spec/filters/json_spec.rb
index 0cc296c..287c192 100644
--- a/spec/filters/json_spec.rb
+++ b/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 {