DataMapper FAQ

Front Matter

Herein lies a list of commonly encountered questions, and perhaps even some answers. This FAQ is intended as a landing page for questions, as well as a location where specific questions answered in #datamapper on freenode, or on the mailing list can be pulled in to be shaped into general answers to problems. Questions and Answers as a result will remain in one of 3 states:

• Unanswered questions.
• Questions answered (either partially or fully) with links or conversations.
• Questions answered with prose here in this document.

The DataMapper irc channel logs can be found on irclogger, and tips and other questions and answers can be found with Alfred. Eventually this document will be used as source material for writing broader documentation for DataMapper, so your questions do matter! Please add questions or answers as you can.

Q) Can you use dm-migrations together with `DataMapper.auto_migrate!` or `DataMapper.auto_upgrade!`?

A) Yes! you can.

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Q) OMG Can I query through a relationship?

A) Yes! Not only can you query through a relationship, you can query through multiple relationships, down to the property level. Example:

Person.all(:pets=>Pet.first(:name=>"Bo", :breed=>"Portuguese Water Dog"))
# => the Obama family
Person.all("pets.breed"=>"Scottish Terrier") # using string based relationship targeting
# => GWB and family
# searching through multiple relationships using properties and query operators.
Building.all("floors.rooms.square_footage.gt"=>1000) 

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Q) Can i set a model class to read only?

There’s at least the dm-is-read_only plugin by postmodern

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Q) Can i set a model to create/insert only?

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Q) I am trying to set a relationship `has n, :bars, :through => :bazes` for my class Foo, but DM says it can’t find model BarFoo when i auto_migrate!"

A) Foo has to have a relationship declared with Baz before you can have a relationship through it.

require 'dm-core'
DataMapper.setup(:default, "sqlite3::memory:")

class Foo
  include DataMapper::Resource
  property :id, Serial
  has n, :bazes, "Baz" # explicitly specifying Baz just to make sure DataMapper doesn't get hung up on inflection
  has n, :bars, :through => :bazes
end

class Bar
  include DataMapper::Resource
  property :id, Serial
  has n, :bazes, "Baz"
  has n, :foos, :through => :bazes
end

class Baz
  include DataMapper::Resource
  property :id, Serial
  belongs_to :foo
  belongs_to :bar
end