Maglev is an alternative Ruby implementation written on top of a Smalltalk VM with an interesting persistence model:
- take an object
- reference it in a global (magic) hash
- commit!
… after which point the object is stored in a repository for easy access by other Maglev Ruby sessions. This provides a very simple mechanism for saving objects, but leaves us with the job of implementing a nice interface to handle the querying and deletion. Persistable is just one of many possible solutions, encapsulated in a module for use with vanilla Ruby classes.
require 'persistable'
class Book < Struct.new(:title, :author)
include Persistable
attr_accessor :read
alias read? read
def to_s
title + ' by ' + author
end
end
book = Book.new("Satan, Cantor, and Infinity", "Raymond Smullyan")
book.persist
# => true
Book.new("Surely You're Joking, Mr. Feynman!", "Richard P. Feynman").persist # => true
Book.new("Green Eggs and Ham", "Dr. Seuss").persist # => true
Book.new("Cat in the Hat", "Dr. Seuss").persist # => true
# Commit instances to the repository
Maglev.commit_transaction
#######################################
# Open a new Maglev session. #
# Book class and instances are #
# already available. #
#######################################
Book.count
# => 4
Book.all
# => [#<struct Book title="Surely You're Joking, Mr. Feynman!", author="Richard P. Feynman">,
#<struct Book title="Cat in the Hat", author="Dr. Seuss">,
#<struct Book title="Green Eggs and Ham", author="Dr. Seuss">,
#<struct Book title="Satan, Cantor, and Infinity", author="Raymond Smullyan">]
# Easily transverse through all our stored books
Book.each {|book| puts book if book.author[/Seuss/]}
# => Cat in the Hat by Dr. Seuss
# => Green Eggs and Ham by Dr. Seuss
# Wonder which Dr. Seuss books I have in my collection
Book.select {|book| !book.author[/Seuss/]}
# => [#<struct Book title="Surely You're Joking, Mr. Feynman!", author="Richard P. Feynman">,
#<struct Book title="Satan, Cantor, and Infinity", author="Raymond Smullyan">]
# Delete books with a short title
Book.delete_if {|book| book.title.length < 15}
# => #<Set: {#<struct Book title="Surely You're Joking, Mr. Feynman!", author="Richard P. Feynman">,
#<struct Book title="Green Eggs and Ham", author="Dr. Seuss">,
#<struct Book title="Satan, Cantor, and Infinity", author="Raymond Smullyan">}>
book.persistent? # => true
# Let's take out our book from the collection
book.desist
book.persistent? # => false
book.transient? # => true
# Commit changes to the repository (or rollback with Maglev.abort_transaction)
Maglev.commit_transaction