maybe "rows: 1", it isn't an option for text_field (but for textarea)
Yes, it is only a text area option, and also the second parameter that a text_field accepts it’s the prompt value (default value) of the field, and that second parameter needs to be a string.
Thank you for the reply 
The problem occurred after a “bundle update” and we did not check what got updated…
The problem comes from https://github.com/bcardarella/client_side_validations, a plugin that we used.
Here is the problem : with the “:validate => true”, “f” seemed to expect fields post-“bundle update” that it did not check pre-“bundle update”.
So i removed “:validate => true” from the following code
<%= form_for([@grade,@tree,@knowledge_cell], :validate => true ) do |f| %>
…
<% end %>
And it worked…
this solution is more about suppressing the problem that to solve it properly… Should check the repo !