9: Advanced Active Record in Java

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9: Advanced Active Record
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class User < ActiveRecord::Base scope :delinquent, lambda { where('timesheets_updated_at < ', 1weekago)}
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Invoke scopes as you would class methods
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>> Userdelinquent => [#<User id: 2, timesheets_updated_at: "2010-01-07 01:56:29">]
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911 Scope Parameters
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You can pass arguments to scope invocations by adding parameters to the lambda you use to define the scope query
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class BillableWeek < ActiveRecord::Base scope :newer_than, lambda { |date| where('start_date > ', date) }
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Then pass the argument to the scope as you would normally
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BillableWeeknewer_than(Datetoday)
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912 Chaining Scopes
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One of the beauties of scopes is that you can chain them together to create complex queries from simple ones:
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>> Timesheetunderutilizedsubmitted => [#<Timesheet id: 3, submitted: true, total_hours: 37
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Scopes can be chained together for reuse within scope definitions themselves For instance, let s say that we always want to constrain the result set of underutilized to submitted timesheets:
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class Timesheet < ActiveRecord::Base scope :submitted, where(:submitted => true) scope :underutilized, submittedwhere('total_hours < 40')
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913 Scopes and has many
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In addition to being available at the class context, scopes are available automatically on has many association attributes
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>> u = Userfind 2 => #<User id: 2, login: "obie"> >> utimesheetssize => 3 >> utimesheetsunderutilizedsize => 1
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91 Scopes
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You can use Arel s join method to create cross-model scopes For instance, if we gave our recurring example Timesheet a submitted_at date attribute instead of just a boolean, we could add a scope to User allowing us to see who is late on their timesheet submission
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scope :tardy, lambda { joins(:timesheets) where("timesheetssubmitted_at <= ", 7daysago) group("usersid") }
Arel s to_sql method is useful for debugging scope definitions and usage
>> Usertardyto_sql => "SELECT users* FROM users INNER JOIN timesheets ON timesheetsuser_id = usersid WHERE (timesheetssubmitted_at <= '2010-07-06 15:27:05117700') GROUP BY usersid" # query formatted nicely for the book
Active Record
Note that as demonstrated in the example, it s a good idea to use unambiguous column references (including table name) in cross-model scope definitions so that Arel doesn t get confused
915 Scope Combinations
Our example of a cross-model scope violates good object-oriented design principles: it contains the logic for determining whether or not a Timesheet is submitted, which is code that properly belongs in the Timesheet class Luckily we can use Arel s merge method (aliased as &) to fix it First we put the late logic where it belongs, in Timesheet:
scope :late, lambda { where("timesheetsubmitted_at <= ", 7daysago) }
Then we use our new late scope in tardy:
scope :tardy, lambda { joins(:timesheets)group("usersid") & Timesheetlate }
If you have trouble with this technique, make absolutely sure that your scopes clauses refer to fully qualified column names (In other words, don t forget to prefix column names with tables) The console and to_sql method is your friend for debugging
9: Advanced Active Record
916 Default Scopes
There may arise use cases where you want certain conditions applied to the finders for your model Consider our timesheet application has a default view of open timesheets we can use a default scope to simplify our general queries
class Timesheet < ActiveRecord::Base default_scope :where(:status => "open") end
Now when we query for our Timesheets, by default the open condition will be applied:
>> Timesheetallmap(&:status) => ["open", "open", "open"]
Default scopes also get applied to your models when building or creating them, which can be a great convenience or a nuisance if you are not careful In our previous example, all new Timesheets will be created with a status of open
>> => >> => Timesheetnew #<Timesheet id: nil, status: "open"> Timesheetcreate #<Timesheet id: 1, status: "open">
You can override this behavior by providing your own conditions or scope to override the default setting of the attributes
>> => >> => Timesheetwhere(:status => "new")new #<Timesheet id: nil, status: "new"> Timesheetwhere(:status => "new")create #<Timesheet id: 1, status: "new">
There may be cases where at runtime you want to create a scope and pass it around as a first class object leveraging your default scope In this case, Active Record provides the scoped method
>> => >> => timesheets = Timesheetscopedorder("submitted_at DESC") [#<Timesheet id: 1, status: "open"] timesheetswhere(:name => "Durran Jordan") []
There s another approach to scopes that provides a sleeker syntax, scoping, which allows the chaining of scopes via nesting within a block
>> Timesheetorder("submitted_at DESC")scoping do >> Timesheetsall >> end => #<Timesheet id: 1, status: "open">