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Posts match “ rails ” tag:

事後 eager load associations

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Eager loading 是 Rails 解決 N+1 問題的方法,用 includes 方法就能在讀取資料庫資料時順便把 association 也讀進來。

不過 includes 是得在讀取之前就先下的指令,而我卻有時候卻只有一堆已經讀進來的 Active Record 物件,想要只用一次 sql query 把每個物件自己的 association 都讀進來,是該怎樣作呢?其實對一個陣列的 ActiveRecord 我們可以這樣作:

posts = [a, b, c] # some AR Post objects, :comments).run()

Rails 4.1 upgrade and FactoryGirl.create with association

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I was trying to upgrade from Rails 4.0 to Rails 4.1. Some specs broke, and I found a small behaviour change.

So my user has_one profile.

user.association(:profile).loaded? # false
FactoryGirl.create(:profile, user: user)
user.association(:profile).loaded? # false

Prior to my upgrade, the profile association will not be loaded before/after FactoryGirl.create call.

However after Rails 4.1

user.association(:profile).loaded? # false
FactoryGirl.create(:profile, user: user)
user.association(:profile).loaded? # true

This means I have to be careful to not have stale profile in tests, calling reload more often.

P.S. my factory_girl gem has remain unchanged in version 4.4.0 and factory_girl_rails gem in 4.4.1. So this is solely due to Rails behavior change.

Apply SEO-friendly url only to show path

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Rails developers often use to_param to add more information to the url. Taking this very page as an example:

We can clear get an idea of what this page is about. Therefore search engines favours this kind of urls.

However, if we just override to_param, we would also see other urls getting changed. One most notable example would be edit path:

I think this can cause issues as the seo-friend param can potentially get very long. Then it would be difficult to notice the edit in the url. Also it may not be that useful to do SEO on action urls (rather than the content urls). We probably will never want edit page to appear on search engines.

I think instead, we should conditionally do SEO on urls that needs it. One way to do this is to have our own version of the url helpers. For example, we can define post_url and post_path like this:

# Override default generated Rails route helpers.
module CustomUrlHelper
  def post_path(target, options = nil)
    target = "#{}-#{post.title[0..60]}" if target.respond_to?(:title)
    super(target, options)
Rails.application.routes.url_helpers.send(:include, CustomUrlHelper)

This means only show/delete/update paths will be SEO friendly. Other paths (especially custom actions) will be unaffected.

Always use respond_to

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We have a controller action which has a error handling state.

  if error
    flash[:error] = render_to_string(partial: "foo_error").html_safe
    render :error # renders error.js.erb

However eventhough the js is rendered, the response content-type is still "text/html". (The request Accept is set to
*/*;q=0.5, text/javascript, application/javascript, application/ecmascript, application/x-ecmascript
) This is very puzzling to us, because this always worked for us.

Later we found out that the root cause was because we call render_to_string. If we remove that call, Rails will again be able to guess the content-type as text/javascript. Somewhere in the Rails internal must have set the type when render_to_string is called, eventhough it is not directly used by the response rendering.

Using respond_to would also solve this issue, forcing Rails to return text/javascript as the content type.

  if error
    flash[:error] = render_to_string(partial: "foo_error").html_safe
    respond_to do |format|
      format.js { render :error }

I guess we should always specify respond_to, so the content-type can be deterministic.

AdequateErrors - Overcoming limitation of Rails model errors API

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Over the years I encountered many issues related to ActiveModel::Errors API. After looking at the Rails source, I realized the original design was the root cause. errors was originally just a hash of array of String, which worked for simple requirements, but not for more complex ones.

In April I started collecting use cases, and study Rails source. Last month I finally put my hands on implementing a solution: a gem to apply object-oriented design principles to make each error an object, and provide new set of APIs to access these objects. I call it AdequateErrors.

AdequateErrors can be accessed by calling model.errors.adequate. It co-exists with existng Rails API, so nothing will break. But what issues does it solve? Let me list them one by one:

Query on errors using where

Imagine we need to access the empty error on any attributes:

model.errors.details.each {|attribute, errors|
  errors.find {|h|
    h[:error] == :empty

AdequateErrors provides a where method. Now we can stop using loops, and write complex queries:

model.errors.adequate.where(type: :empty)
model.errors.adequate.where(attribute: :title, type: :empty)
model.errors.adequate.where(attribute: :title, type: :empty, count: 3)

This returns an array of Error objects. Simple.

Access both the message and details of one particular error

If one attribute has two foo_error and one bar_error, e.g.:

# model.errors.details

{:name=>[{error: :foo_error, count: 1}, {error: :bar_error}, {error: :foo_error, count: 3}]}

How do you access the message on the third particular error? With the current implementation, we have to resort to using array indexes:


Or we can call generate_message to recreate a message from the details, but that's also tedious.

With AdequateErrors, we won't have this problem. Error is represented as an object, message and details are its attributes, so accessing those are straightforward:

e = model.errors.adequate.where(attribute: :title, type: :foo_error).first
e.message # full message

e.options # similar to details, where meta informations such as `:count` is stored.

Lazily evaluating message for internationalization

@morgoth mentioned this issue that when you're adding error, it's translated right away.

# actual:

I18n.with_locale(:pl) { user.error.full_messages } # => outputs EN errors always

# expecting:

I18n.with_locale(:pl) { user.error.full_messages } # => outputs PL errors

I18n.with_locale(:pt) { user.error.full_messages } # => outputs PT errors

Taking this into consideration, AdequateErrors lazily evaluates messages only when message is called.

Error message attribute prefix

Not all error messages start with the attribute name, but Rails forces you to do this. People have developed hacks to bypass this. Others simply assigned errors to :base instead of the actual attribute. This is ugly.

Here is AdequateErrors' solution. It has its own namespace in the locale file, and instead of the global default format "%{attribute} %{message}", the prefix is moved into each individual entries:




      invalid: "%{attribute} is invalid"

      inclusion: "%{attribute} is not included in the list"

      exclusion: "%{attribute} is reserved"

All built-in error types have been converted into this. If one wishes to have prefix-less error, simply have its entry in locale file without the %{attribute}.

Just less error prune code

I remember when I first learned about Object-Oriented design principle in uni, there was this example of payroll system. In the system, one array stores account name and another array stores account number. Whenever we need to delete an account, we need to manipulate both arrays. Further more, if we need to add a new attribute, we need to add a third array. It is very clear that objectifying this system can make it simpler and less error-prone.

This is what the current Rails errors implementation looks like:

def copy!(other) # :nodoc:
  @messages = other.messages.dup
  @details  = other.details.dup

def clear

def delete(key)
  attribute = key.to_sym

This being similar to the case I mentioned above, really can benefit from an object-oriented approach.


If you are a long-time Rails developer, chances are you have met similar issue before, please try this gem. If you have other usecases that you wish to improve on, I would like to know and see if it can be added into the gem. Happy hacking!