Rails 3错误检查关联的存在

Question

I has a model "modela" that has a has_many_through relationship with model "submodelb". In a controller I want to check if modela has any submodelb associated with it. I have tried the two code examples below; however, the both throw the error "undefined method `submodelbs'" if modela does not have any submodelbs. Please help me see what I am doing wrong.

Sample 1: if !@modela.submodelbs.nil?
@submodelbs = @modela.submodelbs
else @submodelbs = [] end

Sample 2: if !@modela.submodelbs.empty?
@submodelbs = @modela.submodelbs
else @submodelbs = [] end

Answer 1

You can use .present? which is the opposite of blank?

@submodelbs = @modela.submodelbs.present? ? @modela.submodelbs : []

But I think your problem is that @modela may be nil or you may have not defined associations correctly in the model.

Answer 2

The reader method produced by has_many_through always returns something that looks like an Array, so it should never return nil. So, can't you just return @modela.submodelbs always?

Answer 3

I use blank?

unless @modela.submodelbs.blank?
   #modela has submodelbs
end

but error messege suggests that you may have something wrong with association definition...

Also what you are trying to achieve can be done with one-liner

@modela.submodelbs ||= []

if submodelbs are nil empty array will be assigned.

Answer 4

为什么不只将支票放在begin...rescue...end块中？