Ruby Metaprogramming ActiveRecord

Question

Suppose I have two models that are similiar. One is called Primer3Template which holds a subset of the fields on Primer3Parameter.

I would like puts only the fields from Primer3Template which exist in Primer3Parameter, as well as upcase the field name and add some string interpolation. I have the code partially working, like this:

> t = Primer3Template.new
> @primer3_parameter = Primer3Parameter.where(:batch_detail_id => 7146)
> t.attributes.each  do | name, value |
>     puts "#{name.upcase}="
>   end

So far so good, this outputs:

ID=
TYPE=
USER_ID=
BATCH_DETAIL_ID=
GENERATED=
STATUS=
P3_FILE_TYPE=
....etc

Next I am trying to output the value of the field from the @primer3_parameter variable, which has the same name as the current name being processed by the loop. I have tried this:

> t.attributes.each  do | name, value |
>     puts "#{name.upcase}=#{@primer3_parameter.name}"
>   end

But this is outputting the name of the class, which is not what I want:

ID=Primer3Parameter
TYPE=Primer3Parameter
USER_ID=Primer3Parameter
BATCH_DETAIL_ID=Primer3Parameter
GENERATED=Primer3Parameter

I want it to skip some fields, and print the value of the field, something like this:

BATCH_DETAIL_ID=7146
GENERATED=true
STATUS=active
P3_FILE_TYPE=Type2

I also tried this:

>  t.attributes.each  do | name, value |
> x = "@primer3_parameters.first.#{name}"
> puts "#{name.upcase}=#{x}"
> end

Which gave me:

ID=@primer3_parameters.first.id
TYPE=@primer3_parameters.first.type
USER_ID=@primer3_parameters.first.user_id
BATCH_DETAIL_ID=@primer3_parameters.first.batch_detail_id
GENERATED=@primer3_parameters.first.generated

ChuckE · Accepted Answer · 2013-08-03 13:21:37Z

2

This:

@primer3_parameters = Primer3Parameter.where(:batch_detail_id => 7146)

returns a collection association. If you want to traverse the Primer3Parameter associated records you have to iterate through them. Like this:

@primer3_parameters.each do |primer3_parameter|
  primer3_parameter.attributes.each do |name, value|
  .....

and the rest is how you did it before.

answered Aug 3, 2013 at 13:21

ChuckE

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2 Comments

port5432 Over a year ago

Hi @ChuckE , thanks for your answer. I would like to traverse the t variable (Primer3Template) and use the name to retrieve the value of the same field on Primer3Parameter. I am thinking instance_variable_get will do it but I can't quite get the syntax right.

ChuckE Over a year ago

but your @primer3_parameter variable is not a Primer3Parameter instance, it is a collection of many.

Ismael · Accepted Answer · 2013-08-03 21:16:50Z

1

dont use eval for this. You might get hurt!

Here is a good alternative

attribute_names = Primer3Template.accessible_attributes
@primer3_parameter = Primer3Parameter.where(:batch_detail_id => 7146)
attribute_names.each do |attr_name|
  puts "#{attr_name.upcase}=#{@primer3_parameter.read_attribute(attr_name)}"
end

Instead of @primer3_parameter.read_attribute(attr_name) I think you could do @primer3_parameter[attr_name] but I'm not sure if there are any drawbacks

answered Aug 3, 2013 at 21:16

Ismael

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Comments

port5432 · Accepted Answer · 2013-08-03 13:52:13Z

0

Got it!!!

> t = Primer3Template.new
> p = Primer3Parameter.where(:batch_detail_id => 7146).first
> t.attributes.each do |name, value|
>     x = eval("p.#{name}")
>     puts "#{name.upcase}=#{x}"
>   end

Output

ID=15
TYPE=Primer3Parameter
USER_ID=
BATCH_DETAIL_ID=7146
GENERATED=false
STATUS=ready
P3_FILE_TYPE=settings
P3_FILE_ID=
P3_FILE_FLAG=false
P3_COMMENT=
SEQUENCE_EXCLUDED_REGION=
SEQUENCE_FORCE_LEFT_END=-1000000
SEQUENCE_FORCE_LEFT_START=-1000000

Did I mention I love Ruby? I hope this helps someone else.

EDIT - Improved Solution

Doesn't requiring instantiating the template class
Only returns the accessible fields which is exactly what I wanted

.

 Primer3Template.accessible_attributes.each do |name|
      x = eval("p.#{name}")
      puts "#{name.upcase}=#{x}"
 end

edited Aug 3, 2013 at 13:52

answered Aug 3, 2013 at 13:43

port5432

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6 Comments

epsilon Over a year ago

Maybe you can use x = p.send(name) to process a Dynmamic Dispatch instead of using eval

epsilon Over a year ago

` Primer3Template.accessible_attributes.each do |name| x = p.send(name) puts "#{name.upcase}=#{x}" end` Object#send

epsilon Over a year ago

The trick was not really an answer to your question, just a way to avoid eval function. My goal is reached if improvement is useful ;)

Peter Alfvin Over a year ago

Do you want all the accessible attribute names from Primer3Template printed out, even when there isn't a corresponding accessible attribute in Primer3Parameter?

port5432 Over a year ago

@Peter Alfvin yes that's exactly what I want. Primer3Parameter is a superset of Primer3Template (both models are mapped to the same physical table via STI)

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