The Status Feed in Java

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The Status Feed
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Listing 1243 Improving from_users_followed_by
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app/models/micropostrb
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class Micropost < ActiveRecord::Base default_scope :order => 'micropostscreated_at DESC' # Return microposts from the users being followed by the given user scope :from_users_followed_by, lambda { |user| followed_by(user) } private # Return an SQL condition for users followed by the given user # We include the user's own id as well def selffollowed_by(user) followed_ids = userfollowingmap(&:id)join(", ") where("user_id IN (#{followed_ids}) OR user_id = :user_id", { :user_id => user }) end end
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Since the conditions on the from_users_followed_by scope are rather long, we have defined an auxiliary function to handle it:
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def selffollowed_by(user) followed_ids = userfollowingmap(&:id)join(", ") where("user_id IN (#{followed_ids}) OR user_id = :user_id", { :user_id => user }) end
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As preparation for the next step, we have replaced
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where(" OR user_id = ", user)
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with the equivalent
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where(" OR user_id = :user_id", { :user_id => user })
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12: Following Users
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The question mark syntax is fine, but when we want the same variable inserted in more than one place, the second syntax, using a hash, is more convenient
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Box 121 Percent paren
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The code in this section uses the Ruby percent-parentheses construction, as in
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%(SELECT followed_id FROM relationships WHERE follower_id = :user_id)
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You can think of %() as equivalent to double quotes, but capable of making multiline strings (If you need a way to produce a multiline string without leading whitespace, do a Google search for ruby here document ) Since %() supports string interpolation, it is particularly useful when you need to put double quotes in a string and interpolate at the same time For example, the code
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>> foo = "bar" >> puts %(The variable "foo" is equal to "#{foo}")
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produces
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The variable "foo" is equal to "bar"
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To get the same output with double-quoted strings, you would need to escape the internal double quotes with backslashes, as in
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>> "The variable \"foo\" is equal to \"#{foo}\""
In this case, the %() syntax is more convenient since is gets you the same result without the explicit escaping
The above discussion implies that we will be adding a second occurrence of user_id in the SQL query, and indeed this is the case We can replace the Ruby code
followed_ids = userfollowingmap(&:id)join(", ")
with the SQL snippet
followed_ids = %(SELECT followed_id FROM relationships WHERE follower_id = :user_id)
The Status Feed
(See Box 121 for an explanation of the %() syntax) This code contains an SQL subselect, and internally the entire select for user 1 would look something like this:
SELECT * FROM microposts WHERE user_id IN (SELECT followed_id FROM relationships WHERE follower_id = 1) OR user_id = 1
This subselect arranges for all the set logic to be pushed into the database, which is more efficient21 With this foundation, we are ready for the final feed implementation, as seen in Listing 1244
Listing 1244 The final implementation of from_users_followed_by
app/models/micropostrb
class Micropost < ActiveRecord::Base default_scope :order => 'micropostscreated_at DESC' # Return microposts from the users being followed by the given user scope :from_users_followed_by, lambda { |user| followed_by(user) } private # Return an SQL condition for users followed by the given user # We include the user's own id as well def selffollowed_by(user) followed_ids = %(SELECT followed_id FROM relationships WHERE follower_id = :user_id) where("user_id IN (#{followed_ids}) OR user_id = :user_id", { :user_id => user }) end end
21 For a more advanced way to create the necessary subselect, see the blog post Hacking a subselect in ActiveRecord
12: Following Users
The Home page with a working status feed
This code has a formidable combination of Rails, Ruby, and SQL, but it does the job, and does it well22
1234 The New Status Feed
With the code in Listing 1244, our status feed is complete As a reminder, the code for the Home page appears in Listing 1245; this code creates a paginated feed of the relevant microposts for use in the view, as seen in Figure 122023 Note that the paginate
22 Of course, even the subselect won t scale forever For bigger sites, you would probably need to generate the feed asynchronously using a background job Such scaling subtleties are beyond the scope of this tutorial, but the Scaling Rails screencasts are a good place to start 23 In order make a prettier feed for Figure 1220, I ve added a few extra microposts by hand using the Rails console