has_and_belongs_to_many confusion

gistfile1.txt

class Poi < ActiveRecord::Base
  attr_accessible :activities
  has_and_belongs_to_many :activities
end

class Activity < ActiveRecord::Base
  attr_accessible :pois
  has_and_belongs_to_many :pois
end

# none of these are working, all because of errors along these lines:
#
# Failure/Error: @poi1.activities.create({:activity_id => @activity1.id })
#     ActiveModel::MassAssignmentSecurity::Error:
#       Can't mass-assign protected attributes: activity_id

@poi1.activities.create({:activity_id => @activity1.id })
@poi1.activities.create({:activity => @activity1 })
@poi1.activities.create(activity: @activity1)
@poi1.activities << @activity1

migrations

class AddActivitiesPoisJoinTable < ActiveRecord::Migration
  def change
    create_table :activities_pois do |t|
      t.integer :activity_id
      t.integer :poi_id
      t.timestamps
    end

    add_index :activities_pois, [:activity_id, :poi_id]
    add_index :activities_pois, [:poi_id, :activity_id]
  end
end