I think this this is a pretty stupid question but I just confused myself
I have several has_many defined. I can reference them in an instance method by saying self.------- If I try to reference via #------ I get nil. Is it just that because it's an ActiveRecord object that it's not available as an instance variable inside the class?
They are not set as instance variables, they are methods. Same as for your properties, you should always access them like this:
self.property
self.read_attribute(:property)
self.write_attribute(:property, value)
They could even be instance variables at the end, but this is an implementation detail and you should always call the code over the well known interface, which is the methods.
associations are methods not instance variables.
Related
I've been working with Dart for a few weeks now, and so far so good. I've run into a problem, however, when trying to access variables from a class that's a few levels higher. I'm not sure how best to explain this without an actual sample, so here's a quick example:
http://pastebin.com/r2ru6G2w
To put this as simply as possible:
AClass has a variable named "parameter."
BClass inherits from AClass, also has a constructor parameter (incomingParameter) that is assigned to the "parameter" variable from AClass.
CClass inherits from BClass, also has a constructor parameter (incomingParameter) that is simply passed on to BClass via the super initializer.
DClass inherits from CClass, but does not have any constructor parameter. Instead, I'm trying to pass the "parameter" variables all the way from AClass into the super initializer.
This results in an error, that says "Only static members can be accessed in initializers." Fair enough. Any idea how to get around this?
Thanks for all the help,
MR
I was looking through the docs for instance_variable_set and saw that the sample code given does this:
obj.instance_variable_set(:#instnc_var, "value for the instance variable")
which then allows you to access the variable as #instnc_var in any of the class's instance methods.
I'm wondering why there needs to be a colon : before the #instnc_var. What does the colon do?
My first instinct is to tell you not to use instance_variable_set unless you really know what you are using it for. It's essentially a tool for metaprogramming or a hack to bypass visibility of the instance variable.
That is, if there is no setter for that variable, you can use instance_variable_set to set it anyway. It's far better if you control the code to just create a setter.
Try looking at accessors for the vast majority of your instance variable setting needs. They are an idiomatic way to have getters and setters for your instance variables "for free" without you having to write those functions.
If you really do need instance_variable_set, it allows the first argument to be a symbol which is the name of the instance variable to set. A colon is part of the Ruby language that is like a "symbol literal": when you type :foo, you've created a symbol with value foo, just like when you type "bar" directly into your code, you create a string literal with value "bar".
I've read that ruby objects are just places where we can store instance variables (and class pointers). So:
class Person
def initialize(age)
#age = age
end
end
Now if we run:
p = Person.new(101)
Then we get:
#<Person:0x86cf5b8 #age=101>
Great, the property age is stored as an instance variable, as expected. But things work a little differently if we convert the model to inherit from ActiveRecord. Now, after instantiating an new Person, we see this:
# timestamps removed
#<Person id: 1, age: 101 >
The age property no longer appears to be an instance variable. So what is really going on here?
I know that we can access the #attributes instance variable, which contains a hash of all the properties and values, so I'm wondering if ActiveRecord is possibly modifying the console output to present the objects attributes in this way.
Is it possible to instantiate a Ruby object where properties are held as attributes and not instance variables, without using ActiveRecord?
Yes, you can extend a ruby class with include ActiveModel::AttributeMethods to expose your instance variables as ActiveModel-like attributes.
See the docs for more information.
as you see in your code 'storing' properties as instance vars was your own doing, so if you wanna hold them any other way, is also up to you. ruby gives you convenience class methods to define getter and setter methods like attr_accessor.
also worth noting, that if you inherit from ActiveRecord::Base, you should not override initialize.
I'm wondering if ActiveRecord is possibly modifying the console output to present the objects attributes in this way.
Well, kind of. The method responsible for that is inspect, and it's implemented by Object in a way that (emplasis mine):
...shows the object's class name, an encoding of the object id, and a list of the instance variables and their values (by calling inspect on each of them).
There's more right after:
User defined classes should override this method to provide a better representation of obj.
This is exactly what ActiveRecord does and the reason why you're seeing this output. The overridden method does not list instance variables, but displays AR attributes.
So just because you aren't seeing the instance variable in the console doesn't mean it isn't there!
I am getting confused as to how to properly set variables in a initializer, I want these to be class level variables, not instance.
And I also want to then create a single instance of another object (it is a connection object, which already has connection pooling built in, so I just need a single reference to it).
My initializer /initializers/my_class.rb
yml = YAML.load_file("#{Rails.root}/config/my_class.yml")
MYMODULE::MyClass.init(yml)
And here is my my_class.rb:
module MYMODULE
class MyClass
def self.init(yml)
#post_url = yml["defaults"]["post_url"]
end
def self.post_url
#post_url
end
# this should be a single instance
def connection_pool
# ???
end
end
end
These class level variables, how can I access them from both class methods and instance methods?
I'm getting wierd behaviour, and I'm confused as to how to reference the post_url from inside of either class methods and instance methods.
I have seen the following ways, unsure which is correct:
self.class.post_url
MyClass.post_url
#post_url
post_url
self.post_url
self.class.post_url or MyClass.post_url will work. The difference is how they work for subclasses (in the former case, subclasses will use their own version of this variable automatically, in the latter, they would share the variable in MyClass).
There is no way to directly access class instance variables from an instance: you have to call a class method which returns (or sets) them. See also: cattr_accessor.
That said, if this is really a singleton, it seems a little strange to me that you would configure part of it on the class, and then reference that info in the (single) instance. Wouldn't it make more sense just to configure this stuff on the instance? Or use a module as a singleton and not create an instance at all?
I'm just learning rails and have noticed that when I create an object that inherits from ActiveRecord::Base (i.e. from a model I migrated), the instance variables in the object do not have a # symbol in front of them.
Is this a rails thing, or did I misunderstand something while learning ruby?
Thanks in advance for your help.
Rails doesn't use individual instance variables to store field data. Instead it makes certain methods available to you which set the correct variables. It helps Rails better populate models when using finds and allows other methods that improve how dynamic Rails is.
When accessing the "instance variables" of your object, you're actually interacting with the getter/setter methods defined by rails which in turn interact with the real instance variables.
This is actually very useful as it allows you to override them when required to modify the behaviour of the variables within your classes.
The columns of your model are not stricto sensu instance variables.
You have access to their getter/setter but they are by nature different: they are meant to be persisted.
Rails defines getter/setter for all model attributes.
Getter/setter can ben declared with attr_accessor function.
class Foo
attr_accessor :bar
def do_something
self.bar=2
#bar=2 # does the same as above
end
end