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I have following db schema:

Seminars (id, ...)
has_many :seminars_presentations
has_many presentations, :through => seminar_presentations

Presentations (id, ...)
has_many :seminar_presentations
has_many :seminars, :through => seminar_presentations

seminars_presentations (seminar_id, presentation_id,......)
belongs_to :presentation
belongs_to :seminar

Now, I would like to implement as below:
Assumption: Presentations & Seminars tables has few records.

I need to show the proposed (future) Seminar dates as links.
To register an exisintg presentation to the seminar, the user clicks
on a future date link. Then already registered presentations for that
(just selected) seminar should be displayed and there should be option
to add one more presentation to it.
Once the user selectes a new presentation to register and clicks the
"Register Presentation" link, that presentation should get registered
with the seminar (from seminar date we can reach the seminar_id and
use it to register).

I have used Query string to get the proposed (future) Seminar date and
then that date I use it to display on the view.

Now, my problem is when I try to add a presentation to the seminar,
rails gives error So, now I cannot register the presentation
to the seminar.

Please help me.