Collection Select With Condition in Rails

Question

I have two models association looks like :

User has_one Office

Office belongs_to User

For Attrubutes looks like :

User : id, name

Office : id, user_id, address

Scenario for add new an office and select user looks like :

<%= f.select :user_id, options_for_select([["Choose User", "0"]] + User.all.collect {|x| [x.name, x.id] if Office.find_by_user_id(x.id).nil? == true }), {}%>

If the user have an office, the user doesn't display in select tag.

Problem :

I have one user 'A', User 'A' appear on select tag when add new an office.

<select id="office_user_id" name="office[user_id]"><option value="0">Choose User</option>
<option value="1">A</option></select>

When I add new an office again, select tag have blank option.

<select id="office_user_id" name="office[user_id]"><option value="0">Choose User</option>
<option></option></select>

When I have two user : user 'A' haven't an office and user 'B' too.

<select id="office_user_id" name="office[user_id]"><option value="0">Choose User</option>
<option value="1">A</option>
<option value="2">B</option></select>

User 'A' have an office and user 'B' haven't

<select id="office_user_id" name="office[user_id]"><option value="0">Choose User</option>
<option></option>
<option value="2">B</option></select>

User 'B' have an office and user 'A' haven't

<select id="office_user_id" name="office[user_id]"><option value="0">Choose User</option>
<option value="1">A</option>
<option></option></select>

User 'A' have an office and user 'B' too

<select id="office_user_id" name="office[user_id]"><option value="0">Choose User</option>
<option></option>
<option></option></select>

etc.

Answer 1

users = User.includes(:office)

It will fire only two queries.

f.select :user_id, options_for_select(users.collect {|x| [x.name, x.id] if x.office.nil?}.compact), prompt: "Please select"

Or you can use join.