The Merb Way: Models
Models are related data and algorithms that respectively represent the properties and behaviors of domain objects used within an application. Models are used in nearly every decently complex object-oriented application because they organize application data in object form. Within the scope of a typical Merb application, examples of modeled objects may include users, posts, entries, or comments. These objects are most often persisted in a database through the use of an Object Relational Mapper, or ORM. As agnosticism has been central to Merb’s design, application developers are free to integrate whatever Ruby ORM they may need. Nonetheless, the default ORM included as part of the standard Merb stack is DataMapper, an ActiveRecord-like ORM that aims to push the boundaries of object-data interaction even more significantly toward the object side.
Given its acceptance as part of the standard Merb stack, we will cover the use of only DataMapper in this chapter. However, don’t let that stop you from using ActiveRecord, Sequel, or any of the other options, since each of these ORMs is capable of creating, retrieving, updating, and deleting persisted data and is fully supported by the Merb core.
Prior to being able to use the DataMapper ORM within your Merb application, you will have to make sure that you have done three things:
- Included the DataMapper dependencies within config/dependencies.rb
- Selected DataMapper as the ORM in init.rb
- Configured your database connection in config/database.yml
However, if you used merb-gen to generate a standard application and have SQLite3 installed, then you’ll find that all of these are already covered for you. If your application was generated in some other manner, you may want to see Chapter 1 to figure out how your configuration files should be laid out.
DataMapper works with several different database back ends. Support for MySQL, PostgreSQL, and SQLite3 are all included via the DataMapper core. If you’re using another database application, you may be able to find an appropriate adapter in the dm-more or dm-adapters gems. Alternatively, if your computer has the memory for it, there’s a DataMapper adapter that will store your model records completely in memory.
Repository configuration is done in a YAML file named config/database.yml. The standardly generated config/database.yml file is broken into four environments—development, test, production, and rake—corresponding to the environments that are standardly used by a Merb application. If you have another environment—for instance, a staging environment—it, too, can be listed in this file. Typically there is overlap in the configuration of environments, so YAML node anchors and aliases are used to reduce redundancy.