If not passed a variable, use DB default (Ruby on Rails)

Question

I have a function that is inserting a record into my DB (MySQL). It has many columns, many of which have default values in the DB. Passing in values for these variables is therefore optional.

def assign_X_to_Y( options = {} )
 . . . 
 @bar.var1 = options[:foo]
 . . .
end

I would like to do the following:

-If a variable exists (ex: options[:foo]), add it to the record I'm making.

@bar.var1 = options[:foo]

-If it doesn't, I don't want to add it--I want to use the DB default.

I know I can simply do an if:

if options[:foo]
   @bar.var1 = options[:foo]
end

But I have a lot of these variables and so I think there must be a nicer way that having loads of if-statements. Something like the "if doesn't exist set to null" expression:

@bar.var1 = options[:foo] || nil

Is there anything like what I am saying? I can't use the above expression because I don't want to set it to null (which I think it would do), I want to use the default value…

Thanks in advance!

Are you assigning the values to a model or creating a SQL statement? Please add the assign_X_to_Y method to your question. Its impossible to give refactoring advice about an method which is not even shared with the reader. — max
– max, Commented Jun 23, 2015 at 21:29
Added @bar.var1 = options[:foo] to the method…does that clarify that I am adding values to a model? — Jo.P
– Jo.P, Commented Jun 24, 2015 at 3:28
Yeah, if its a model just do @bar.assign_attributes(options) — max
– max, Commented Jun 24, 2015 at 4:07

max · Accepted Answer · 2015-06-23 22:06:47Z

1

If @bar is an model you can simply pass a hash:

Bar.create(hash) # creates a Bar with the defaults from your schema
@bar.assign_attributes(hash)
@bar.update(hash) # same as object but commits the changes to the db

If bar is a Plain Old Ruby class you can give it the same functionality by:

class Bar

  attr_accessor :foo
  attr_accessor :baz
  attr_accessor :woggle

  def initialize(hash)
     assign_values(hash)
  end

  def assign_attributes(hash)
    assign_values(hash)
  end 

  private 

  def assign_values(hash)
    hash.each do |k,v|
      send "#{k}=", v
    end
  end
end

Then I can simply create an object with:

Bar.new(foo: 1, baz: 3)

Note that this will respect object encapsulation - if I try to do:

Bar.new(haxxored: true)

It will raise a NoMethodError. Just like @bar.haxxored = true.

edited Jun 23, 2015 at 22:06

answered Jun 23, 2015 at 21:59

max

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Comments

Collin Graves · Accepted Answer · 2015-06-23 21:34:57Z

0

If I'm understanding your question correctly, the best way to handle this would be to use the public_send method in Ruby:

def set_new_property(obj, prop_name, prop_value)
  obj.class.__send__(attr_accessor: "#{prop_name}")
  obj.public_send("#{prop_name}=", prop_value)
end

Bear in mind that you'll have to set explicit attribute accessors for each potential property being assigned.

edited Jun 23, 2015 at 21:34

answered Jun 23, 2015 at 21:29

Collin Graves

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1 Comment

max Over a year ago

Or you could just do instance_variable_set if you are going to trump encapsulation anyways.

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