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CHAPTER 4 WORKING WITH A DATABASE: ACTIVE RECORD
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Here, we stored the object that find returned in the local variable event. Then we were able to interrogate it and ask for its attributes. Now, all this worked because an event with the id 1 actually exists. If instead we had searched for a record that we know doesn t exist (say, 1037), Active Record would have raised an exception.
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>> Event.find(1037) Active Record::RecordNotFound: Couldn't find Event with ID=1037 from active_record/base.rb:1035:in `find_one' from active_record/base.rb:1018:in `find_from_ids' from active_record/base.rb:419:in `find' from (irb):5
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Active Record raises a RecordNotFound exception and tells us it couldn t find any events with the id of 1037. Of course it couldn t. We know that no such record exists. The lesson here is that you use find(:id) when you re looking for a specific record that you expect to exist. If the record doesn t exist, it s probably an error you want to know about; therefore, Active Record raises RecordNotFound.
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RECOVERING FROM RECORDNOTFOUND ERRORS
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When you use find with a single id, you expect the record to exist. But often you re not in control of the input, such as when it comes from a URL. While the path /events/show/1 might happily return a result, what s stopping Joe User from changing the parameters (say, to /events/show/1037) in an attempt to mess with your application You probably don t want to raise an exception every time something like this happens. You want to recover gracefully. So, how can you recover gracefully from a RecordNotFound exception should you need to You can use Ruby s facility for error handling: begin and rescue. Here s how this works:
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begin Event.find(1037) rescue ActiveRecord::RecordNotFound puts "We couldn't find that record" end
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First, we open a begin block. Then, we cause a RecordNotFound error by deliberately searching for a record that we know doesn t exist. When the error occurs, Ruby runs the code we put inside the rescue part of the body, which prints a friendly message.
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CHAPTER 4 WORKING WITH A DATABASE: ACTIVE RECORD
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Finding a Single Record Using :first
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You can find the first record the database returns using find(:first). This will always return exactly one item, unless the table is empty, in which case, nil is returned.
>> Event.find(:first) => #<Event:0x2247874 @attributes={"title"=>"Tiki Party", "url"=>nil, "occurs_on"=>"2007-09-02", "id"=>"1", "description"=>nil, "location"=>"Hampton's Apartment"}>
Keep in mind that this isn t necessarily the first record in the table. It actually depends on the database software you re using. It s the equivalent of saying SELECT * FROM table LIMIT 1 in SQL. If you ever need to find a record and don t particularly care which record it is, find(:first) can come in handy. Note that find(:first) doesn t raise an exception if the record can t be found.
Finding All Records
So far, we ve looked at finding a single record. In each case, find returned a single Event object. But what if you want to find more than one event In our application, we ll want to display all the events on the homepage. If you provide find with the symbol :all, it returns all records for that class.
>> events = Event.find(:all) => [#<Event:0x223aad4>, #<Event:0x223aa98>, #<Event:0x223aa0c>, #<Event:0x223a980>]
The square brackets in the response indicate that find has returned an array. We can confirm this by asking the events variable what its class is.
>> events.class => Array
Sure enough, events tells us it s an Array. To be precise, it s an array of Event objects. Like all Ruby arrays, we can ask for its size.
>> events.size => 4
Since events is an array, we can access the individual elements it contains by using its index, which is numeric, starting at 0.
>> events[0] => #<Event:0x223aad4 @attributes={"title"=>"Tiki Party", "url"=>nil, "occurs_on"=>"2007-09-02", "id"=>"1", "description"=>nil, "location"=>"Hampton's Apartment"}>
CHAPTER 4 WORKING WITH A DATABASE: ACTIVE RECORD
And once we ve isolated a single Event object, we can get at its attributes.
>> events[0].title => "Tiki Party"
What s happening here is that find(:all) produced an array, and we accessed the object at the 0 index and called the title method. We can also use the first method, which all arrays respond to, and get the same result, but with a little more natural syntax.
>> events.first.title => "Tiki Party"
If we wanted to iterate over the collection, we could use the each method, which, again, works with all arrays. Here, we ll loop over the array, extract each item into a variable called event, and print its location attribute using the puts command.
>> events.each {|event| puts event.location } Hampton's Apartment Portland, Oregon Flaming Moe's Poznan, Poland
Sometimes you want your results ordered. For example, if you re listing all your events, you probably want them listed chronologically. The find method also accepts an argument that defines on which column or columns to order the results. It s called :order, and for you SQL heroes, it corresponds to the SQL ORDER clause.
>> events = Event.find(:all, :order => 'occurs_on') => [#<Event:0x250b7e0...>, #<Event:0x250b27c...>, #<Event:0x250a804...>] >> events.each {|event| puts event.occurs_on } 2007-04-14 2007-05-17 2007-09-02 2007-10-13
By default, any column that is ordered is in ascending order (for example, 1 10, or a z). If you want to reverse this to get descending order, use the DESC modifier (the same way you would in SQL, since the value of :order is really just a SQL fragment).
>> events = Event.find(:all, :order => 'occurs_on DESC') => [#<Event:0x24dfdd4...>, #<Event:0x24dfdac...>, #<Event:0x24dfd84...>] >> events.each {|event| puts event.occurs_on } 2007-10-13 2007-09-02 2007-05-17 2007-04-14
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