the problem of the implementation of last chapter is:
1. it has a line of code:
following_ids = user.following_ids
this will fetch all the followed users of this user,
but what we need to just if user_id is included in this set.
SQL already optimized things like this, check inclusion in a set of another table.
2. the original way will pull out all the microposts and stick them into an array.
although they are paginated in the view, but in the memory, there is still an array holding all these data.
what we need is it honestly pull out 30 elements at a time.
so we will solve the two problems in this chapter.
1. the solution to both problems involves converting the feed from a class method to a scope
scope is a rails method for restricting database selects based on certain conditions.
for example, to arrange for a method to select all admins, we can add a scope:
scope :admin, where(:admin => true)
then
User.admin
will return all admin users.
2. I know you will ask what's the difference of using scope and class method:
when using independently, they are the same, both pull all admin users from database.
but when using in chains, it will be different:
User.admin.paginate(:page => 1)
this will only pull 30 admins from the database at a time.
so the database did some optimization here, the paginate will happen in the database level.
cool, isn't.
this will make a very bid differences for big site.
3. now it is time to change the from_users_followed_by method using scope:
scope :from_users_followed_by, lambda { |user| followed_by(user) }
note, this scope need to take an argument, so we have to use lambda, a anonymous function.
and the method is a little complicated, so we put it into a method, followed_by(user.)
def self.followed_by(user) following_ids = user.following_ids where("user_id in (#{following_ids}) or user_id = :user_id", {:user => user}) end
but this only make sure the database don't pull all micropost, but it still pull out all followed users.
we should think of a way to replace
following_ids = user.following_ids
we need to use sql text here:
following_ids = %(select followed_id from relationships where follower_id = :user_id)
so the new method is:
def self.followed_by(user) following_ids = %(SELECT followed_id FROM relationships WHERE follower_id = :user_id) where("user_id IN (#{following_ids}) OR user_id = :user_id", { :user_id => user }) end
so the final sql being passed to database is:
SELECT * FROM microposts WHERE user_id IN (SELECT followed_id FROM relationships WHERE follower_id = 1) OR user_id = 1
the database will be more efficient when executing this.
when Micropost.from_users_followed_by(user), it will still return all micropost.
but when chained with paginate or other things, it will be much different.
Micropost.from_users_followed_by(user).paginate(:page => 1)
it will not get all followed users, it just get enough to get the first 30 micropost
4. a way to construct a string: %(.........)
%(fjdkfjdlsjlfdsfjdlsfjlds
fjdsfjdslfdjs)
this is a way to construct string, you can think of it the same as double quotes.
the use case of %() to construct a string is:
a. when you want to show a multiline string.
b. when you want to have "" or "#{jfldjs}" inside a string.
>> puts %(The variable "foo" is equal to "#{foo}".)
all " in this string will be just display, no need to escape,
without %(), it will have to be:
>> "The variable \"foo\" is equal to \"#{foo}\"."
this is not very readable.
发表评论
-
12.3 the status feed
2011-10-30 15:34 8601. we need to get all the micro ... -
12.2 a working follow button with Ajax
2011-10-29 18:10 9141. in the last chapter, in the ... -
12.2 a web interface for following and followers.
2011-10-28 22:14 8801.before we do the UI, we need ... -
12. following user, 12.1 relationship model
2011-10-18 14:29 7551. we need to use a relationshi ... -
11.3 manipulating microposts.
2011-10-17 15:31 9011. since all micropost actions ... -
11.2 show microposts.
2011-10-17 12:01 7051. add test to test the new use ... -
11.1 user micropost -- a micropost model.
2011-10-17 10:43 11081. we will first generate a mic ... -
10.4 destroying users.
2011-10-16 15:47 746in this chapter, we will add de ... -
10.3 showing users list
2011-10-15 20:41 775in this chapter, we will do use ... -
10.2 protect pages.
2011-10-15 15:11 668again, we will start from TD ... -
10.1 updating users.
2011-10-14 18:30 7101. git checkout -b updating-use ... -
9.4 sign out
2011-10-13 15:21 737whew!!!, last chapter is a long ... -
9.3 sign in success.
2011-10-12 15:39 7511. we will first finish the cre ... -
9.1 about flash.now[:error] vs flash[:error]
2011-10-12 15:37 727There’s a subtle difference ... -
9.2 sign in failure
2011-10-12 12:19 660start from TDD!!! 1. requir ... -
9.1 sessions
2011-10-12 10:00 645a session is a semi-permanent c ... -
what test framework should you use?
2011-10-11 16:56 0for integration test, i have no ... -
what test framework should you use?
2011-10-11 16:56 0<p>for integration test, ... -
8.4 rspec integration tests
2011-10-11 16:53 723in integration test, you can te ... -
8.3 sign up success
2011-10-11 14:39 782Chapter 8.3 this part, we will ...
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