It sounds like you should establish an association between the two models:
class Client < ActiveRecord::Base has_many :details end
class Detail < ActiveRecord::Base belongs_to :client end
This would require a client_id column in the details table.
Then, when the form is submitted, you would do something like:
client = Client.new(params[:client]) client.details.build(params[:detail]) client.save
The one save inserts both the clients and details rows.
HTH