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CHAPTER 4 WORKING WITH A DATABASE: ACTIVE RECORD
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Deleting Records
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We re finally at the last component of CRUD: delete. When you re working with databases, you inevitably need to delete records. If a user cancels her order, or if a book goes out of stock, or even if you have an error in a given row, you might want to delete it. Sometimes you need to delete all rows in an entire table, and sometimes you want just to delete a specific row. Active Record makes deleting rows every bit as easy as creating them. There are two styles of row deletion: destroy and delete. The destroy style works on the instance. It instantiates the object, which means that it finds a single row first, and then deletes it from the database. The delete style operates on the class, which is to say it operates on the table rather than a given row from that table.
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Using destroy
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The easiest and most common way to remove a record is to use the destroy method, which means the first thing we need to do is find the record we want to destroy.
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>> event = Event.find(1) >> event.destroy
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For those interested, the SQL that Active Record generates in response to the destroy operation is as follows.
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DELETE FROM events WHERE id = 1;
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As a result, the event with the id of 1 is permanently destroyed. But we still have the object hanging around in the variable event, so how can it really be gone The answer is that while the object remains hydrated (which is to say that it retains all its attributes), it s frozen. You can still access its attributes, but you can t modify them. Let s see what happens if we try to change the location.
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>> event.location = 'foobar' TypeError: can't modify frozen hash
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So, it appears that the now deleted event is a frozen hash. While the object remains, it is read-only, so we can t modify it. Given this fact, if we re just going to be deleting the record, we don t really need to create an explicit Event object after all. We can do the destroy in a one-line operation.
>> Event.find(1).destroy
Here, the object instantiation is implicit. We re still calling the destroy instance method, but we re not storing an Event object in a local variable first.
CHAPTER 4 WORKING WITH A DATABASE: ACTIVE RECORD
We can still do better. We can use the class method destroy, which does a find automatically. Just like find and create, you can use destroy directly on the class (that is, you don t create an object first). Since it operates on the table and not the row, you need to help it out by telling it which row or rows you want to target. Here s how we would delete the event with the id 1:
>> Event.destroy(1)
Sometimes you want to destroy more that one record. Just as with find, you can give destroy an array of primary keys whose rows you want to remove. You use the square brackets ([]) to indicate that you re passing an array.
>> Event.destroy([1,2,3])
Even though ActiveRecord::Base.destroy is a class method, it does, in fact, instantiate each object before destroying it. We can tell this by looking at its source.
def destroy(id) id.is_a (Array) id.each {|id| destroy(id) } : find(id).destroy end
Here, we can see that if the received argument is an array, destroy will iterate over the array and call the same destroy method once for each item in the array. This will effectively cause it to take the else path of the conditional, which performs a find first (instantiating the object), and then calls the instance version of destroy. Neat, huh That pretty much covers destroy.
Using delete
The second style of row deletion is delete. Every Active Record class has class methods called delete and delete_all. The delete family of methods differ from destroy in that they don t instantiate or perform callbacks on the object they re deleting. They remove the row immediately from the database. Just like find and create, you use delete and delete_all directly on the class (that is, you don t create an object first). Since it operates on the table and not the row, you need to help it out by telling it which row or rows you want to target. Here s how it looks:
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