Q:

Set local: true as default for form_with in Rails 5

I'm working on a project where we won't be using ajax calls for submitting the forms, so I need to put local: true in every form in the project, as indicated in the rails docs:
:local - By default form submits are remote and unobstrusive XHRs.  Disable remote submits with local: true. 
Is there any way to set the local option as true by default? 
We're using Rails 5 form_with helper like this:
<%= form_with(model: @user, local: true) do |f| %>
    <div>
        <%= f.label :name %>
        <%= f.text_field :name %>
    </div>

    <div>
        <%= f.label :email %>
        <%= f.email_field :email %>
    </div>
    <%= f.submit %>
<% end %>

A:

As you've stated it can be set on a per form basis with local: true.  To set it globally use the configuration option form_with_generates_remote_forms
https://guides.rubyonrails.org/configuring.html - config.action_view.form_with_generates_remote_forms determines whether form_with generates remote forms or not.  This defaults to true. 
Set it in an initializer:
# config/initializers/action_view.rb
Rails.application.config.action_view.form_with_generates_remote_forms = false

A:

Consider overriding the form_with method:
# form_helper.rb
def form_with(options)
  options[:local] = true
  super options
end

That should solve it for every form in your application. 

A:

The Rails configurations can be set in config/application.rb file. 
module App
  class Application < Rails::Application
    # [...]

    config.action_view.form_with_generates_remote_forms = false
  end
end

Guy C answer is good, but it's more idiomatic to put all config in this file rather than a separate initializer; That's where most of the Rails dev would expect it. 
Note that this would spell disaster if you put it config/development.rb only or other env specific files.