how to add records to has_many :through association in rails

I think you can simply do this:

 @cust = Customer.new(params[:customer])
 @cust.houses << House.find(params[:house_id])

Or when creating a new house for a customer:

 @cust = Customer.new(params[:customer])
 @cust.houses.create(params[:house])

You can also add via ids:

@cust.house_ids << House.find(params[:house_id])

'The best way' depends on your needs and what feels most comfortable. Confusion comes from differences ActiveRecord's behavior of the new and create methods and the << operator.

The new Method

new will not add an association record for you. You have to build the House and Agent records yourself:

house = @cust.houses.new(params[:house])
house.save
agent = Agent(customer_id: @cust.id, house_id: house.id)
agent.save

Note that @cust.houses.new and House.new are effectively the same because you need to create the Agent record in both cases.

The << Operator

As Mischa mentions, you can also use the << operator on the collection. This will only build the Agent model for you, you must build the House model:

house = House.create(params[:house])
@cust.houses << house
agent = @cust.houses.find(house.id)

The create Method

create will build both House and Agent records for you, but you will need to find the Agent model if you intend to return that to your view or api:

house = @cust.houses.create(params[:house])
agent = @cust.agents.where(house: house.id).first

As a final note, if you want exceptions to be raised when creating house use the bang operators instead (e.g. new! and create!).