gistfile1.rb

require 'rubygems'
require 'dm-core'

DataMapper::Logger.new(STDOUT, :debug)
DataMapper.setup(:default, 'sqlite3::memory:')

class Person
  include DataMapper::Resource
  property :id, Serial
  has n, :projects
  has n, :tasks, :through => :projects
  # everything works fine if specified like this
  # has n, :tasks, :through => :projects, :via => :tasks
end

class Project
  include DataMapper::Resource
  property :id, Serial
  belongs_to :person
  has n, :tasks
end

class Task
  include DataMapper::Resource
  property :id, Serial
  belongs_to :project
end

DataMapper.auto_migrate!

p '-'*80

# use it when you create a child and have the parent object at hand
person  = Person.create
project = Project.new(:person => person, :tasks => [ Task.new ])
project.save

# use it in queries
puts Task.all(:project => project).inspect


# Have a look at CREATE TABLE projects, it creates a task_id there which definitely isn't right
# As a consequence, the creation of a new Project fails because there is no (unneeded) task_id present

#  ~ (0.000143) SELECT sqlite_version(*)
#  ~ (0.000105) DROP TABLE IF EXISTS "people"
#  ~ (0.000017) DROP TABLE IF EXISTS "projects"
#  ~ (0.000015) DROP TABLE IF EXISTS "tasks"
#  ~ (0.000024) PRAGMA table_info("people")
#  ~ (0.000361) CREATE TABLE "people" ("id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT)
#  ~ (0.000009) PRAGMA table_info("projects")
#  ~ (0.000113) CREATE TABLE "projects" ("id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, "person_id" INTEGER NOT NULL, "task_id" INTEGER NOT NULL)
#  ~ (0.000126) CREATE INDEX "index_projects_task" ON "projects" ("task_id")
#  ~ (0.000096) CREATE INDEX "index_projects_person" ON "projects" ("person_id")
#  ~ (0.000009) PRAGMA table_info("tasks")
#  ~ (0.000107) CREATE TABLE "tasks" ("id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, "project_id" INTEGER NOT NULL)
#  ~ (0.000097) CREATE INDEX "index_tasks_project" ON "tasks" ("project_id")
# "--------------------------------------------------------------------------------"
#  ~ (0.000031) INSERT INTO "people" DEFAULT VALUES
#  ~ projects.task_id may not be NULL (code: 19, sql state: , query: INSERT INTO "projects" ("person_id") VALUES (1), uri: sqlite3://:memory:)
# /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.10.0/lib/dm-core/adapters/data_objects_adapter.rb:163:in `execute_non_query': projects.task_id may not be NULL (DataObjects::IntegrityError)
#   from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.10.0/lib/dm-core/adapters/data_objects_adapter.rb:163:in `execute'
#   from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.10.0/lib/dm-core/adapters/data_objects_adapter.rb:267:in `with_connection'
#   from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.10.0/lib/dm-core/adapters/data_objects_adapter.rb:161:in `execute'
#   from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.10.0/lib/dm-core/adapters/data_objects_adapter.rb:59:in `create'
#   from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.10.0/lib/dm-core/adapters/data_objects_adapter.rb:32:in `each'
#   from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.10.0/lib/dm-core/adapters/data_objects_adapter.rb:32:in `create'
#   from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.10.0/lib/dm-core/repository.rb:129:in `create'
#   from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.10.0/lib/dm-core/resource.rb:698:in `hookable___create_nan_before_advised'
#   from /opt/local/lib/ruby/gems/1.8/gems/extlib-0.9.13/lib/extlib/hook.rb:299:in `_create'
#   from /opt/local/lib/ruby/gems/1.8/gems/extlib-0.9.13/lib/extlib/hook.rb:297:in `catch'
#   from /opt/local/lib/ruby/gems/1.8/gems/extlib-0.9.13/lib/extlib/hook.rb:297:in `_create'
#   from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.10.0/lib/dm-core/resource.rb:608:in `save_self'
#   from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.10.0/lib/dm-core/resource.rb:374:in `save!'
#   from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.10.0/lib/dm-core/resource.rb:358:in `save'
#   from dm-features.rb:34