From: Brian Candler <b.candler@...>
Date: 2011-01-04T23:44:06+09:00
Subject: Re: Validates is exists

Jose tomas R. wrote in post #972073:
> Event has_and_belongs_to_many :users
> User  has_and_belongs_to_many :events
>
> Im trying to verify that user doesnt signup more than one time
...
>       if @event.users.find(@user) != @user.id

Foo.find(x) looks for a record with id X, and will raise an exception if 
it doesn't exist, as you already discovered.

Completely untested, but you could try some variation of

    if @event.users.find_by_id(@user.id)

    if @event.users.where(:id => @user.id).count > 0

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