how to generate barcode in c# net with example >> Event.delete(1) => 1 in Font

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>> Event.delete(1) => 1
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Here, we specified a single primary key for the event we wanted to delete. The operation responded with the number of records that were removed. Since a primary key uniquely identifies a single record, only one record was deleted.
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CHAPTER 4 WORKING WITH A DATABASE: ACTIVE RECORD
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Just as with find, you can give delete an array of primary keys whose rows you want to delete. You use the square brackets ([]) to indicate that you re passing an array.
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>> Event.delete([1,2,3]) => 3
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Note Unlike find, which is capable of collecting any arguments it receives into an array automatically,
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delete must be supplied with an array object explicitly. So, while Model.find(1,2,3) will work, Model.delete(1,2,3) will fail with an argument error (because it s really receiving three arguments). To
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delete multiple rows by primary key, you must pass an actual array object. The following will work, since it s a single array (containing three items), and thus, a single argument: Model.delete([1,2,3]).
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Deleting with Conditions
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You can delete all rows that match a given condition with the delete_all class method. The following will delete all events before a certain date.
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>> Event.delete_all("ends_at < '2007-01-01'") >> 5
The return value of delete_all is the number of records that were deleted.
Caution If you use delete without any arguments, it will delete all rows in the table, so be careful! Most
of the time, you ll pass it a string of conditions.
When Good Models Go Bad
So far, we ve been very nice to our models and have been making them happy by providing them with just the information that they need. But we know that in the previous chapter, we provided validations that were preventing us from saving bad records to the database. Specifically, we told the Event model that it should never allow itself to be saved to the database if it isn t provided a title and location. Taking a look at the Event model, shown in Listing 4-1, you can recall how validations are specified.
CHAPTER 4 WORKING WITH A DATABASE: ACTIVE RECORD
Listing 4-1. The app/models/event.rb File
class Event < ActiveRecord::Base belongs_to :user validates_presence_of :title, :location end
You may have noticed in our generated scaffolding that we use a helper method called error_messages_for(:event) to print out a helpful little error message. That helper isn t black magic; it s simply a bit of code that asks the model for its list of errors (also referred to as the errors collection), and returns a nicely formatted block of HTML to show the user.
Note You may have noticed that we call methods in Ruby with the dot (.) For instance, we would say
@user.errors to get the error collection back. However, there is an idiomatic convention in Ruby documentation of using the # symbol, along with the class name, to let the reader know that there is a method it can call on an object. For example, on our Event class, we could use the method @event.title as Event#title, because it s something that acts upon a particular @event, but not the Event class itself. You have also seen that we can write the code Event.count, because we don t need to know about a particular @event, but only Event objects in general. Keep this convention in mind when you re reading Ruby
documentation.
The secret to this is that every Active Record object has an automatic attribute added to it called errors. To get started, let s create a fresh new Event object.
>> event = Event.new => #<Event:0x2536f30 @new_record=true, @attributes={"title"=>nil, "url"=>nil, "occurs_on"=>nil, "description"=>nil, "location"=>nil}> >> event.errors.any => false
This seems odd because we know that this new event should have errors because it s invalid. We didn t give it a title or a location. It turns out that this is because we haven t triggered the validations yet. We can cause them to occur a couple of ways. The most obvious way is to attempt to save the object.
>> event.save => false
Every time we ve used save before, the model has happily chirped true back to us. But this time, save returns false. This is because before the model would allow itself to be saved, it ran through its gauntlet of validations, and one or more of those validations failed.
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