You're reading the documentation for the next release of Hashie, which should be 3.0.1. Please read [UPGRADING](UPGRADING.md) when upgrading from a previous version. The current stable release is [3.0](https://github.com/intridea/hashie/blob/v3.0.0/README.md).
The library is broken up into a number of atomically includeable Hash extension modules as described below. This provides maximum flexibility for users to mix and match functionality while maintaining feature parity with earlier versions of Hashie.
Coercions allow you to set up "coercion rules" based either on the key or the value type to massage data as it's being inserted into the Hash. Key coercions might be used, for example, in lightweight data modeling applications such as an API client:
Value coercions, on the other hand, will coerce values based on the type of the value being inserted. This is useful if you are trying to build a Hash-like class that is self-propagating.
The KeyConversion extension gives you the convenience methods of `symbolize_keys` and `stringify_keys` along with their bang counterparts. You can also include just stringify or just symbolize with `Hashie::Extensions::StringifyKeys` or `Hashie::Extensions::SymbolizeKeys`.
The MethodAccess extension allows you to quickly build method-based reading, writing, and querying into your Hash descendant. It can also be included as individual modules, i.e. `Hashie::Extensions::MethodReader`, `Hashie::Extensions::MethodWriter` and `Hashie::Extensions::MethodQuery`.
This extension can be mixed in to instantly give you indifferent access to your Hash subclass. This works just like the params hash in Rails and other frameworks where whether you provide symbols or strings to access keys, you will get the same results.
A unique feature of Hashie's IndifferentAccess mixin is that it will inject itself recursively into subhashes *without* reinitializing the hash in question. This means you can safely merge together indifferent and non-indifferent hashes arbitrarily deeply without worrying about whether you'll be able to `hash[:other][:another]` properly.
This extension can be mixed in to silently ignore undeclared properties on initialization instead of raising an error. This is useful when using a Trash to capture a subset of a larger hash.
This extension can be mixed in to provide for safe and concise retrieval of deeply nested hash values. In the event that the requested key does not exist a block can be provided and its value will be returned.
Though this is a hash extension, it conveniently allows for arrays to be present in the nested structure. This feature makes the extension particularly useful for working with JSON API responses.
Mash is an extended Hash that gives simple pseudo-object functionality that can be built from hashes and easily extended. It is designed to be used in RESTful API libraries to provide easy object-like access to JSON and XML parsed hashes.
**Note:** The `?` method will return false if a key has been set to false or nil. In order to check if a key has been set at all, use the `mash.key?('some_key')` method instead.
Dash is an extended Hash that has a discrete set of defined properties and only those properties may be set on the hash. Additionally, you can set defaults for each property. You can also flag a property as required. Required properties will raise an exception if unset.
Clash is a Chainable Lazy Hash that allows you to easily construct complex hashes using method notation chaining. This will allow you to use a more action-oriented approach to building options hashes.
Essentially, a Clash is a generalized way to provide much of the same kind of "chainability" that libraries like Arel or Rails 2.x's named_scopes provide.
Rash is a Hash whose keys can be Regexps or Ranges, which will map many input keys to a value.
A good use case for the Rash is an URL router for a web framework, where URLs need to be mapped to actions; the Rash's keys match URL patterns, while the values call the action which handles the URL.
If the Rash's value is a `proc`, the `proc` will be automatically called with the regexp's MatchData (matched groups) as a block argument.
**Note:** The Rash is automatically optimized every 500 accesses (which means that it sorts the list of Regexps, putting the most frequently matched ones at the beginning).