RWTH Aachen, Computer Science Student triAGENS GmbH, Developer moon lum moonbeamlabs by Lucas Dohmen Active Record vs. Data Mapper Patterns for Object Document Mappers

Ashikawa • Ruby adapter for Aran oDB • But how?

NoSQL Object Relational Mappers

So I sat down to think… Source: http://www.fotocommunity.de/pc/pc/display/24290075

Active Record

“An object that wraps a row in a database table or view, encapsulates the database access, and adds domain lo ic on that data. Martin Fowler in Patterns of Enterprise Application Architecture (2003)

Domain Model • It is somethin from the domain • It encapsulates data • It adds business lo ic • It may validate the data

• Constructs an instance from an SQL result • Constructs a new instance for later insertion • Updates the databases • Wraps commonly used SQL queries In ActiveRecord it also…

# The problem for example in Rails‘ ActiveRecord # (Inspired by solnic) require "active_record" class ClassWithActiveRecord < ActiveRecord::Base; end (ClassWithActiveRecord.methods - Object.methods).count # => 391 # It consists of 77 modules

It does too much.

…and it is hard to test it

• You could also have multiple classes accessin one collection dependin on the data • Or nested attributes • This does not fit very well in that pattern NoSQL specific problems

Data Mapper

Warnin for Ruby Developers • You‘ve been lied to • DataMapper 1 implements ActiveRecord • DataMapper 2 will be the first implementation of DataMapper for Ruby

“A layer of Mappers that moves data between objects and a database while keepin them independent of each other and the mapper itself. Martin Fowler in Patterns of Enterprise Application Architecture (2003)

“ Good desi n is about drawin lines Uncle Bob in Architecture the Lost Years (Ruby Midwest 2011)

Domain Models Persistence

Domain Models Persistence

“The Data Mapper is a layer of software that separates the in-memory objects from the database. Martin Fowler in Patterns of Enterprise Application Architecture (2003)

This is especially useful for NoSQL… • because the Mapper can choose the class dependin on the data • … or create multiple objects

It is also interestin … • … because the database does not validate • This is solely the responsibility of the Domain Model • We are separatin concerns

“ With Data Mapper the in-memory objects needn’t know even that there’s a database present Martin Fowler in Patterns of Enterprise Application Architecture (2003)

…which simplifies unit testin

# Examples from Datamapper 2 # again from solnic # Define a model class User include DataMapper::Model attribute :id, Integer attribute :name, String attribute :age, Integer end # Define a mapper DataMapper.generate_mapper_for(User) do key :id map :name, :to => :username end # Search for data DataMapper[User].all DataMapper[User].order(:age) DataMapper[User].find(age: 23)

• But an attribute of the model could also be another model • This can be used for has_many etc. • … but it is also handy in a Document Store • The model doesn‘t need to care, only the mapper has to handle the difference

“ The database is a detail! Uncle Bob in Architecture, the Lost Years (Ruby Midwest 2011)

• Keeps track of all objects read from the database and their modifications • Handles how updates are made • Instead of the invokin explicit save methods, tell the unit of work to commit • Works reat with Data Mapper • Works reat with batch requests Unit of Work

To conclude…

• ActiveRecord is simpler • Data Mapper is a better fit in a lot of cases • It has better support for denormalization • DataMapper and NoSQL make the database become a detail, simplifyin tests • With a Workin Set you can take full advanta e of batch requests

Ashikawa • Ruby adapter for Aran oDB • Both patterns will be implemented

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