habtm in Rails 3.2

1_create_apples.rb

class CreateApples < ActiveRecord::Migration
  def change
    create_table :apples do |t|
      t.string :name
    end
  end
end

2_create_oranges.rb

class CreateOranges < ActiveRecord::Migration
  def change
    create_table :oranges do |t|
      t.string :name
    end
  end
end

3_create_apples_oranges.rb

class CreateApplesOranges < ActiveRecord::Migration
  def up
    create_table :apples_oranges, :id => false do |t|
      t.belongs_to :apple, :orange
      t.timestamps
    end
  end

  def down
    drop_table :apples_oranges
  end
end

models-apple.rb

class Apple < ActiveRecord::Base
  has_and_belongs_to_many :oranges
end

models-orange.rb

class Orange < ActiveRecord::Base
  has_and_belongs_to_many :apples
end

zee_console

$: r c
Loading development environment (Rails 3.2.0)

>> Apple.create(:name => "foo")

   (0.1ms)  begin transaction
  SQL (0.5ms)  INSERT INTO "apples" ("name") VALUES (?)  [["name", "foo"]]
   (2.6ms)  commit transaction

=> #<Apple id: 1, name: "foo">

>> Orange.create(:name => "bar")

   (0.1ms)  begin transaction
  SQL (0.4ms)  INSERT INTO "oranges" ("name") VALUES (?)  [["name", "bar"]]
   (2.3ms)  commit transaction

=> #<Orange id: 1, name: "bar">

>> Apple.first.oranges << Orange.first

  Apple Load (0.2ms)  SELECT "apples".* FROM "apples" LIMIT 1
  Orange Load (0.2ms)  SELECT "oranges".* FROM "oranges" LIMIT 1
   (0.0ms)  begin transaction
   (0.2ms)  INSERT INTO "apples_oranges" ("orange_id", "apple_id") VALUES (1, 1)
SQLite3::ConstraintException: constraint failed: INSERT INTO "apples_oranges" ("orange_id", "apple_id") VALUES (1, 1)
   (0.1ms)  rollback transaction
ActiveRecord::StatementInvalid: SQLite3::ConstraintException: constraint failed: INSERT INTO "apples_oranges" ("orange_id", "apple_id") VALUES (1, 1)
	from /Library/Ruby/Gems/1.8/gems/sqlite3-1.3.5/lib/sqlite3/statement.rb:108:in `step'
	from /Library/Ruby/Gems/1.8/gems/sqlite3-1.3.5/lib/sqlite3/statement.rb:108:in `each'
	from /Library/Ruby/Gems/1.8/gems/sqlite3-1.3.5/lib/sqlite3/statement.rb:107:in `loop'
	from /Library/Ruby/Gems/1.8/gems/sqlite3-1.3.5/lib/sqlite3/statement.rb:107:in `each'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/connection_adapters/sqlite_adapter.rb:253:in `to_a'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/connection_adapters/sqlite_adapter.rb:253:in `exec_query'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/connection_adapters/abstract_adapter.rb:280:in `log'
	from /Library/Ruby/Gems/1.8/gems/activesupport-3.2.0/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/connection_adapters/abstract_adapter.rb:275:in `log'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/connection_adapters/sqlite_adapter.rb:247:in `exec_query'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/connection_adapters/abstract/database_statements.rb:61:in `exec_insert'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/connection_adapters/abstract/database_statements.rb:88:in `insert'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/connection_adapters/abstract/query_cache.rb:14:in `insert'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/associations/has_and_belongs_to_many_association.rb:29:in `insert_record'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/associations/collection_association.rb:492:in `concat_records'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/associations/collection_association.rb:342:in `add_to_target'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/associations/collection_association.rb:491:in `concat_records'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/associations/collection_association.rb:489:in `each'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/associations/collection_association.rb:489:in `concat_records'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/associations/collection_association.rb:134:in `concat'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/associations/collection_association.rb:149:in `transaction'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/connection_adapters/abstract/database_statements.rb:190:in `transaction'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/transactions.rb:208:in `transaction'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/associations/collection_association.rb:148:in `transaction'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/associations/collection_association.rb:134:in `concat'
	from /Library/Ruby/Gems/1.8/gems/activerecord-3.2.0/lib/active_record/associations/collection_proxy.rb:117:in `<<'
	from (irb):3>> 

@ryanbigg pointed out on Twitter that it's the timestamps on the join table that is tripping it up.

https://twitter.com/#!/ryanbigg/status/161739182664990721