How to ad a "virtual column" to a model

Rails
1

Hi,

is it possible to define a column in a model, which is not present in
the table? this column should return a combination of two other columns.

Model.columnC=Model.columnA+"-"+Model.columnB

A and B are in the table, C isn’t

I was thinking of attr_accessor, but i cant get it to work

any pointers?

2

Remco Hh wrote:

is it possible to define a column in a model, which is not present in
the table? this column should return a combination of two other columns.

Model.columnC=Model.columnA+"-"+Model.columnB

A and B are in the table, C isn’t

If you just want to grab this value, you just need a standard Ruby
method in your model definition:

def columnC
columnA + “-” + columnB
end

If you want to be able to write to this attribute as well, then you’ll
have to write a method along the lines of

def columnC=(new_value)
columnA, columnB = new_value.split("-")
new_value
end

although obviously there is some ambiguity with “strings-like-this”.

Hope that helps,
Chris

3

Have a look at ActiveRecord’s composed_of thingy

Fred

4

any pointers?
You only need to use attr_accessor if you’re looking to set the value
as well as get it. You should just be able to create a method to do
this:

class Cheese < ActiveRecord::Base
def name
origin+" "+type
end
end

my_cheese = Cheese.new
my_cheese.origin = “Kent”
my_cheese.type = “Stilton”
puts my_cheese.name -> “Kent Stilton”

Hope that helps,

Steve

5

No, that doesn’t help
this virtual column needs to be included in the dataset when i do a
find:all query

Stephen B. wrote:

any pointers?
You only need to use attr_accessor if you’re looking to set the value
as well as get it. You should just be able to create a method to do
this:

class Cheese < ActiveRecord::Base
def name
origin+" "+type
end
end

my_cheese = Cheese.new
my_cheese.origin = “Kent”
my_cheese.type = “Stilton”
puts my_cheese.name -> “Kent Stilton”

Hope that helps,

Steve

6

There might be some misunderstaning here, at lest on my part -

The dataset is a group of objects. When you do: model_instance.name –
you’re not really accessing any data, you’re calling a method inside
ActiveRecord, (method_missing, actually)

If you had manually defined a method called ‘name’, there would be no
difference to the code using the model. It just sees a method.

Unless by saying ‘the dataset’, you mean the model’s @attributes
variable?

Cheers
Starr

7

If performing a find all is your goal with the results being the join of
two
tables fields why not just find all for each field and use .join to
combine
their results in a new array. Then just use .each to iterate through
that
array and/ar access it by its indexes? Perhaps a little bit exhaustive
for
your goal but it is an option. Im sure there is something more DRY
though.

8

On 10/9/06, Benjamin G. [email protected] wrote:

If performing a find all is your goal with the results being the join of
two tables fields why not just find all for each field and use .join to
combine their results in a new array. Then just use .each to iterate through
that array and/ar access it by its indexes? Perhaps a little bit exhaustive
for your goal but it is an option. Im sure there is something more DRY
though.

Or write a new find method named FindWithVirtualColumn or some such,
that
writes the actual SQL query you’re trying to emulate. ActiveRecord is
great, but don’t be afraid of SQL when neccessary.

On 10/9/06, Starr [email protected] wrote:

–
===Tanner B.===
[email protected]
http://tannerburson.com <—Might even work one day…