Rails 3 - 在更新之前检查该对象是否对params有效

Question

I have very newbie question. How can i check that object of model is valid with new params BEFORE updating it?

I want transform that:

def update
  @obj = SomeModel.find( params[:id] )

  if @obj.update_attributes( params[:obj] )
    # That have been updated
  else
    # Ups, errors!
  end
end

To something like that:

def update
  @obj = SomeModel.find( params[:id] )

  if @obj.valid_with_new_params( params[:obj] )
    @obj.update_attributes( params[:obj] )
  else
    # Ups, errors!
  end
end

Answer 1

To update the attributes without saving them, you can use

@obj.assign_attributes( params[:obj] )

Then to check if the object is valid, you can call

@obj.valid?

If the object is not valid, you can see the errors (only after calling .valid? ) by calling

@obj.errors

If the object is valid, you can save it by calling

@obj.save

However, all of this usually isn't necessary. If the object isn't valid, then ActiveRecord won't save the object to the database, so all of the attribute changes are forgotten when you leave the controller action.

Also, since an invalid record won't be saved to the database, you can always just call Object.find() again to get the original object back.

Answer 2

You can call the valid? method to run the validations.

This doesn't guarantee that a subsequent save will succeed if some of your validations depend on the state of other objects in the database. Te save could also fail for reasons unconnected to validations (eg a foreign key constraint)

I'm not sure why you'd want this pattern

Answer 3

The object won't be saved if the passed argument doesn't produce a valid object, so you can use your way just fine. You can see the errors (if any) using the @obj.errors array.

Answer 4

update_attributes method validate object and return false if object is invalid. So, you can just write:

if @obj.update_attributes( params[:obj] )
  # That have been update
else
  # Ups, errors!
end

Answer 5

The answer is that you can define a method

def valid_with_new_params(hash)
   self.attributes = hash
   valid?
end

But that would be unnecessary because @obj.update_attributes(params[:obj]) returns true if the obj was successfully updated and false otherwise. Note also that internally the update_attributes method runs all validations on the @obj so that you have @obj.errors available if the update failed.

Answer 6

To update the attributes without saving them

@obj.attributes = params[:obj]   0r
@obj.attributes = {:name => “Rob”}

To then check if the object is valid

@obj.valid?

To check if there is any error

@obj.errors