1.Ruby Object? in c
2.juke box extension
3.memory allocation
4.ruby type system
5.create an exception
6embed a ruby interpreter
7.bridge to other language
8.c api
1.
p269
Sometimes, though, life is more complicated. Perhaps you want to define a global variable
whose valuemust be calculated when it is accessed. You do this by defining hooked
and virtual variables. A hooked variable is a real variable that is initialized by a named
function when the corresponding Ruby variable is accessed. Virtual variables are similar
but are never stored: their value purely comes from evaluating the hook function.
See the API section that begins on page 294 for details.
If you create a Ruby object from C and store it in a C global variable without exporting
it to Ruby, you must at least tell the garbage collector about it, lest ye be reaped
inadvertently.
static VALUE obj;
// ...
obj = rb_ary_new();
rb_global_variable(obj);
You may have noticed that our sample database tables all define an integer
column called id as their primary key. This is an Active Record convention.
“But wait!” you cry. “Shouldn’t the primary key of my orders table be the
order number or some other meaningful column? Why use an artificial
primary key such as id?”
The reason is largely a practical one—the format of external data may
change over time. For example, you might think that the ISBN of a book
would make a good primary key in a table of books. After all, ISBNs are
Report erratum Prepared exclusively for Don Francis
PRIMARY KEYS AND IDS 198
unique. But as this particular book is being written, the publishing industry
in the US is gearing up for a major change as additional digits are
added to all ISBNs.
If we’d used the ISBN as the primary key in a table of books, we’d have to
go through and update each row to reflect this change. But then we’d have
another problem. There’ll be other tables in the database that reference
rows in the books table via the primary key. We can’t change the key in the
books table unless we first go through and update all of these references.
And that will involve dropping foreign key constraints, updating tables,
updating the books table, and finally reestablishing the constraints. All in
all, something of a pain.
If we use our own internal value as a primary key, things work out a lot
better. No third party can come along and arbitrarily tell us to change
things—we control our own keyspace. And if something such as the ISBN
does need to change, it can change without affecting any of the existing
relationships in the database. In effect, we’ve decoupled the knitting
together of rows from the external representation of data in those rows.
Now there’s nothing to say that we can’t expose the id value to our end
users. In the orders table, we could externally call it an order id and print
it on all the paperwork. But be careful doing this—at any time some regulator
may come along and mandate that order ids must follow an externally
imposed format, and you’d be back where you started.
If you’re creating a new schema for a Rails application, you’ll probably
want to go with the flow and give all of your tables an id column as their
primary key. If you need to work with an existing schema, Active Record
gives you a simple way of overriding the default name of the primary key
for a table.
class BadBook < ActiveRecord::Base
set_primary_key "isbn"
end
Normally, Active Record takes care of creating new primary key values
for records that you create and add to the database—they’ll be ascending
integers (possibly with some gaps in the sequence). However, if you override
the primary key column’s name, you also take on the responsibility
of setting the primary key to a unique value before you save a new row.
Perhaps surprisingly, you still set an attribute called id to do this. As far as
As we’ll see later, join tables are not included in this advice—they should not have an id column.
Active Record is concerned, the primary key attribute is always set using
an attribute called id. The set_primary_key declaration sets the name of the
column to use in the table. In the following code, we use an attribute
called id even though the primary key in the database is isbn.
book = BadBook.new
book.id = "0-12345-6789"
book.title = "My Great American Novel"
book.save
# ...
book = BadBook.find("0-12345-6789")
puts book.title # => "My Great American Novel"
p book.attributes #=> {"isbn" =>"0-12345-6789",
"title"=>"My Great American Novel"}
Just to make things more confusing, the attributes of the model object
have the column names isbn and title—id doesn’t appear. When you need
to set the primary key, use id. At all other times, use the actual column
name.
If a model object has an attribute named balance, you can access the
attribute’s value using the indexing operator, passing it either a string or
a symbol. Here we’ll use symbols.
account[:balance] #=> return current value
account[:balance] = 0.0 #=> set value of balance
However, this is
deprecated in normal code, as it considerably reduces
your options should you want to change the underlying implementation
of the attribute in the future. Instead, you should access values or model
attributes using Ruby
accessor methods.
account.balance #=> return current value
account.balance = 0.0 #=> set value of balance
The value returned using these two techniques will be cast by Active
Record to an appropriate Ruby type if possible (so, for example, if the
database column is a timestamp, a Time object will be returned). If you
want to get the raw value of an attribute, append _before_type_cast to the
method form of its name, as shown in the following code.
COLUMNS AND ATTRIBUTES 195
David Says. . .
Overriding Model Attributes
Here’s an example of the benefits of using accessors to get at the
attributes of models. Our account model will raise an exception immediately
when someone tries to set a balance below a minimum value.
class Account < ActiveRecord::Base
def balance=(value)
raise BalanceTooLow if value < MINIMUM_LEVEL
self[:balance] = value
end
end
account.balance_before_type_cast #=> "123.4", a string
account.release_date_before_type_cast #=> "20050301"
Finally, inside the code of the model itself, you can use the read_attribute( )
and write_attribute( ) private methods. These take the attribute name as a
string parameter.
Boolean AttributesSome databases support a boolean column type, others don’t. This makes
it hard for Active Record to abstract booleans. For example, if the underlying
database has no boolean type, some developers use a char(1) column
containing “t” or “f” to represent true or false. Others use integer columns,
where 0 is false and 1 is true. Even if the database supports boolean types
directly (such as MySQL and its bool column type), they might just be
stored as 0 or 1 internally.
The problem is that in Ruby the number 0 and the string “f” are both
interpreted as true values in conditions.4 This means that if you use the
value of the column directly, your code will interpret the column as true
when you intended it to be false.
# DON'T DO THIS
user = Users.find_by_name("Dave")
if user.superuser
grant_privileges
end
4Ruby has a simple definition of truth. Any value that is not nil or the constant false is
true.
To query a column in a condition, you must append a question mark to
the column’s name.
# INSTEAD, DO THIS
user = Users.find_by_name("Dave")
if user.superuser?
grant_privileges
end
This form of attribute accessor looks at the column’s value. It is interpreted
as false only if it is the number zero; one of the strings "0", "f", "false",
or "" (the empty string); a nil; or the constant false. Otherwise it is interpreted
as true.
If you work with legacy schemas or have databases in languages other than
English, the definition of truth in the previous paragraph may not hold.
In these cases, you can override the built-in definition of the predicate
methods. For example, in Dutch, the field might contain J or N (for Ja or
Nee). In this case, you could write
class User < ActiveRecord::Base
def superuser?
self.superuser == 'J'
end
# . . .
end
Storing Structured Data
It is sometimes convenient to store attributes containing arbitrary Ruby
objects directly into database tables. One way that Active Record supports
this is by serializing the Ruby object into a string (in YAML format) and
storing that string in the database column corresponding to the attribute.
In the schema, this column must be defined as type text.
Because Active Record will normally map a character or text column to a
plain Ruby string, you need to tell Active Record to use serialization if you
want to take advantage of this functionality. For example, we might want
to record the last five purchases made by our customers. We’ll create a
table containing a text column to hold this information.
File 6 create table purchases (
id int not null auto_increment,
name varchar(100) not null,
last_five text,
primary key (id)
);
In the Active Record class that wraps this table, we’ll use the serialize( )
declaration to tell Active Record to marshal objects into and out of this
column.
File 8 class Purchase < ActiveRecord::Base
serialize :last_five
# ...
end
When we create new Purchase objects, we can assign any Ruby object to
the last_five column. In this case, we set it to an array of strings.
File 8 purchase = Purchase.new
purchase.name = "Dave Thomas"
purchase.last_five = [ 'shoes', 'shirt', 'socks', 'ski mask', 'shorts' ]
purchase.save
When we later read it in, the attribute is set back to an array.
File 8 purchase = Purchase.find_by_name("Dave Thomas")
pp purchase.last_five
pp purchase.last_five[3]
This code outputs
["shoes", "shirt", "socks", "ski mask", "shorts"]
"ski mask"
Although powerful and convenient, this approach is problematic if you
ever need to be able to use the information in the serialized columns outside
a Ruby application. Unless that application understands the YAML
format, the column contents will be opaque to it. In particular, it will be
difficult to use the structure inside these columns in SQL queries. You
might instead want to consider using object aggregation, described in Section
15.2, Aggregation, on page 247, to achieve a similar effect.