Validate_presence_of strange behaviour


I need some help to understand some strange behaviour when using

I have a form that updates table row. One field is text and I require it
to be completed.

With no validation in place in the model I can obvioulsy update the row
and either put a text entry in or leave it blank and the update happens

After adding validate_presence_of :activity to the model ....
I then update with the text item completed and it updates okay.
but if I then update with the text item null I get what appears to be a
totally unrelated error:-

"Called id for nil, which would mistakenly be 4 -- if you really wanted
the id of nil, use object_id" and which points to the code line:-

<form action="../update/<%= %>" method="POST">

Of course what I would expect is to get an "Activity must be completed"

So when I would expect validate_presence_of to trigger a real validation
error it in fact throws up this other condition.

I suspect the actual error may be unrelated to validate_presence_of but
that the validate routine is merely exposing some other underlying issue
and without the validation in place it all works okay and I dont have
any issues or errors shown!

Confused since the validate_presence_of should be so simple and yet it
throws up this other condition.

Can anyone give me a steer as to what/where I should be investigating

Any help much appreciated!

You get an error in this line

<form action="../update/<%= %>" method="POST">

That has nothing to do with the validation as such.
You try to get which will fail with the error you
mentioned if it's nil
So the problem is wherever you do
@activities = ...

Why this doesn't fail without the validation is another question

By the way:
You use activities here, which is plural and should contain a list
rather then an object.
For easy code reading it would be better to be clear about that
point and either use @activity if it's a singular object or your
code is wrong to try to get an id from it
(But can't say that for sure without the rest of your code)