Ruby on Rails Pitfall

Ruby on Rails PitfallOr just stupid mistakes we made

Robin LuIN-SRC Studio

[email protected]

Friday, May 22, 2009

mailto:[email protected]

mailto:[email protected]

IN-SRC Studio

• http://www.in-src.com

• Team behind Caibangzi.com

• Full stack Ruby On Rails Development

• Projects from Pepboys, Vitality, Healthwise...


http://www.in-src.com

http://www.in-src.com

‘and’ or ‘&&’

What does this mean?

result = func(arg) and render(:text => result)


‘and’ or ‘&&’



Why not this?

result = func(arg) && render(:text => result)


‘and’ or ‘&&’



Why not this?

result = func(arg) && render(:text => result)

Be aware of the operator precedence


strip_tags

Display user input text without tags

What we did:


strip_tags

Whentext = ‘<img title="http://example.com/x.js?" src="#"’

the page becomes:

<p> <img title="http://example.com/x.js?" src="#" </p>


http://www.dofala.com/tmp/cbz/x.js?




strip_tags

strip_tags is not safe by itself

h strip_tags(text)


cache

Controller

class Blog1Controller < ApplicationControllerdef list

unless read_fragment(:action => 'list') @articles = Article.find_recent

end end

end

<% cache do %> <ul> <% for article in @articles -%>

<li><p><%= h(article.body) %></p></li> <% end -%> </ul>

<% end %>

list.html.erb


cache

Result:

sometime got crash due to uninitialized @articles


cachearticle list


cache

check cache

article list


cache

check cache list

article list


cache

check cache list

article list

render


cache

check cache list

article list

render

article new


cache

check cache list

article list

expire cache

render

article new


cache

check cache list

article list

expire cache

render

check cache

article new


cache

check cache list

article list

expire cache

render

check cache

crashed by non-init @articles

article new


cache

• defensive: handle the exception

• postpone init of @articles

• update caches instead of expiring them

Solutions?

none of them is perfect


object id


object id

Check nil? everywhere?


object idconfig.whiny_nil = true


validate_uniqueness_of



We always get errors like this:

A ActiveRecord::StatementInvalid occurred in fund#add_watch_fund:

Mysql::Error: Duplicate entry '1234-271' for key 2: INSERT INTO `watch_funds` (`account_id`, `position`, `fund_id`, `created_at`) VALUES(1234, 19, 271, '2009-05-06 19:13:50')


validate_uniqueness_ofProcess A

Process B



Process B

unique?



Process B

unique? select ....



Process B

unique? select ....unique?



Process B


Insert



Process B


Insert

Insert



Process B


Insert

Insert

crash!



validate_uniqueness_of may not guarantee the uniqueness

use your own lock if the uniqueness is critical to you.


conditions

Background:

• category has many subcategories

• subcategory has many posts

• post belongs to subcategory

we need to select all posts in a category.


conditionsWhat we did:

named_scope :in_category, lambda { |cat| conditions = [cat.subcategories.map {|subcat| 'posts.subcategory_id = ?' }.join(" OR ")] cat.subcategories.each {|subcat| conditions << subcat.id } {:conditions => conditions}}


conditions

Result:

we get all posts when a category has no subcategories


conditionsWhen category has no subcategory

named_scope :in_category, lambda { |cat| conditions = [cat.subcategories.map {|subcat| 'posts.subcategory_id = ?' }.join(" OR ")] cat.subcategories.each {|subcat| conditions << subcat.id } {:conditions => conditions}}


conditions

When you compose conditions, be aware that sometime nothing to

compose means

the conditions should match nothing,

not the conditions should be empty.


before_create

set a flag if the author of the post is an admin

What we did:


before_create

Result:

Only post by admin can be saved


before_create

All these callbacks are Filters

Be careful not to break the filter chain by what you return from the filters!


after_create

send a mail whenever a new record is created

What we did:


after_create

Result:

sometime the record save failed but we still get mail notification


after_create

before_create

create

after_create

all in one transaction

begin......commit

all the steps between this should be transactional


after_create

• send a mail

• delete a file

• expire a cache

What are non-transactional actions?


after_create

• try not put non-transaction actions into transactions.

• after_commit

• in controller


Thanks!


Ruby on Rails Pitfall

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