rails 3.0.10 conditions in select()

i have this select

<%= select("post", "field",@variable.all.collect) {|p| [ p.name, p.id ] }, :include_blank =>'blank')%>            

to generate a dropdown of 4 registers

and i did

{|p| [ p.name, p.id ] if helper_method(p.id) }

expecting this output:

blank
reg4

instead i got this output:

blank



reg4

and i can't figure out how to get the expected output