Active Record - Object Wrapping a Table Row

Rails
1

I’m following through an example provided by DHH in his book, “Agile Web
Development with Rails”. This example deals with a database with an
“orders” table that has column names, “name”, “email”, “address” and
“pay_type”.

The Order class is created thusly:

class Order < ActiveRecord::Base
end

Later the example describes one way of reading data from the rows of the
table as follows:

pos = Order.find(:all, :conditions => “name = ‘Dave’ and pay_type =
‘po’”)

Then, the author says, “The result will be an array of all matching
rows, each neatly wrapped in an Order object.”

It would have been nice if he had gone on to at least describe some of
the characteristics of this particular object. He doesn’t. I have a
feeling that one thing that’s important for us to know about this object
is that the values for each field in the table row is contained in an
instance variable with the name of the column heading. I’m not sure if
the raw name of the column heading is massaged in any way, i.e.,
singular/plural, upper/lower case, etc. I’m also assuming that there
are accessor methods for these instance variables. What I’d like to
know is whether what I have surmised is, in fact, true. I’d also like
to know if there is anything else that I should know about this object.
Can someone please fill in the blanks for me?

TIA any input.

 ... doug
2

On 7 July 2012 17:29, Doug J. [email protected] wrote:

Later the example describes one way of reading data from the rows of the
feeling that one thing that’s important for us to know about this object
is that the values for each field in the table row is contained in an
instance variable with the name of the column heading. I’m not sure if
the raw name of the column heading is massaged in any way, i.e.,
singular/plural, upper/lower case, etc. I’m also assuming that there
are accessor methods for these instance variables. What I’d like to
know is whether what I have surmised is, in fact, true. I’d also like
to know if there is anything else that I should know about this object.
Can someone please fill in the blanks for me?

As it states the result is effectively an array of Order objects. So
to get the name of the first object one uses pos[0].name.
Alternatively one can do things like
pos.each do |p|

here you can access p.name, p.pay_type etc.

end

Colin

3

As it states the result is effectively an array of Order objects. So
to get the name of the first object one uses pos[0].name.

I THINK you are confirming my suspicion. Clearly, the first object
(which corresponds to the first table row) is pos[0]. From that I would
deduce that pos[0].name is a getter method within the pos[0] object
for accessing the name instance variable which apparently contains the
value for the name column in the first object (which corresponds to the
first row).

From my point of view, the omission in the book is that the order object
contains an instance variable for each column and that instance variable
has the same name as the corresponding column. Additionally, the order
object has accessor methods (or, at least a getter method; but, I
suspect that there is also a setter method) for each of these instance
variables.

Anyway, I think that my suspicions are confirmed. Thanks for the input.

  ... doug
4

On 7 July 2012 19:26, Doug J. [email protected] wrote:

From my point of view, the omission in the book is that the order object
contains an instance variable for each column and that instance variable
has the same name as the corresponding column. Additionally, the order
object has accessor methods (or, at least a getter method; but, I
suspect that there is also a setter method) for each of these instance
variables.

I have not looked at that book recently but I would have expected that
to be covered rather early on. I am sure that if you turn over the
page you will find it.

Colin

5

The order object is an Active Record object. You can find some
comprehensive guides on Active Record here:

–
Oscar Del B.
Sent with Sparrow (http://www.sparrowmailapp.com/?sig)

6

On Jul 7, 7:26pm, Doug J. [email protected] wrote:

If you want to be entirely correct, the attributes aren’t stored as
individual instance variables - they’re stored together in a hash.
That is usually irrelevant though - the important thing is the
accessor methods.

Fred

7

On 8 July 2012 19:57, Doug J. [email protected] wrote:

If you want to be entirely correct, the attributes aren’t stored as
individual instance variables - they’re stored together in a hash.
That is usually irrelevant though - the important thing is the
accessor methods.

Nice to know; but, you’re right that we do not really need to know about
the internal workings. As you say, we DO need to know about the
accessor methods and that was the part that I thought was missing from
the book. I have an earlier edition. It may have been picked up by
now.

Make sure that the book you are following is written for the version
of Rails that you are using (which should be at least version 3), at
least to the first two levels (eg 3.2) otherwise you will get
horribly confused as the examples will not work.

You might be better working through railstutorial.org, which is free
to use online.

Colin

8

If you want to be entirely correct, the attributes aren’t stored as
individual instance variables - they’re stored together in a hash.
That is usually irrelevant though - the important thing is the
accessor methods.

Nice to know; but, you’re right that we do not really need to know about
the internal workings. As you say, we DO need to know about the
accessor methods and that was the part that I thought was missing from
the book. I have an earlier edition. It may have been picked up by
now.

Thanks for the input.

… doug