No longer loading json_pure if json has already been loaded.

CHANGELOG.md

@@ -1,6 +1,20 @@
 Unreleased Changes
 ------------------
 
+* Will no longer require json_pure if the json gem has already been loaded.
+  This will result in a warning and a degraded experience if json < 1.8.1
+  has already been loaded.
+
+  Mixing json/pure with json/ext results in json errors, for example:
+
+  ```ruby
+  some_hash = { 'jsonrpc' => 'abc', 'jsonversion' => 1 }
+  some_hash.to_json
+  #=> raises a JSON::Pure::Generator::State TypeError
+  ```
+
+  [See related GitHub issue #20](https://github.com/jmespath/jmespath.rb/issues/20).
+
 1.2.2 (2016-03-31)
 ------------------
 

lib/jmespath.rb

@@ -1,12 +1,16 @@
-begin
-  # Attempt to load the native version if available, not availble
-  # by default for Ruby 1.9.3.
-  gem('json', '>= 1.8.1')
-  require 'json/ext'
-rescue Gem::LoadError
-  # Fallback on the json_pure gem dependency.
-  gem('json_pure', '>= 1.8.1')
-  require 'json/pure'
+if Object.const_defined?(:JSON) && JSON::VERSION < '1.8.1'
+  warn("WARNING: jmespath gem requires json gem >= 1.8.1; json #{JSON::VERSION} already loaded")
+else
+  begin
+    # Attempt to load the native version if available, not availble
+    # by default for Ruby 1.9.3.
+    gem('json', '>= 1.8.1')
+    require 'json/ext'
+  rescue Gem::LoadError
+    # Fallback on the json_pure gem dependency.
+    gem('json_pure', '>= 1.8.1')
+    require 'json/pure'
+  end
 end
 
 require 'stringio'
@@ -28,7 +32,6 @@ module JMESPath
 
   class << self
 
-
     # @param [String] expression A valid
     #   [JMESPath](https://github.com/boto/jmespath) expression.
     # @param [Hash] data