Adding a 'fallback' argument to Model#get

[shiloa]

Motivation

Sometimes it's useful to retrieve some default value from a Model, even if that value doesn't exist in the db (or not even an actual model attribute, if waiting implementation).

This fallback value isn't persisted, but rather acts as a sane default when the actual value returns true on _.isEmpty( value ). I found it useful in view templates, when I don't want to inline a lot of ternary logic like:

<%= @user.get('name') || 'Anonymous' %>

<!-- compared to -->

<%= @user.get('name', 'Anonymous') %>

It is basically equivalent to Python's dict.get(key, default=<something>) or Ruby's Hash#fetch(key, [default]). I do realize Model#get is a core method, so I'm not taking this addition lightly.

I've searched existing issues, but couldn't find something similar. Apologies in advance if I missed one. I did my best to stick to the guidelines, so hopefully things are in order.

Thanks!

[gsamokovarov]

JavaScript has some pretty common idioms for those cases - just use this.user.get('name') || 'Anonymous') if you want to avoid the this.user.has call.

For CoffeeScript you can use the above idiom or (@user.get 'name') ? 'Anonymous' and it will even test against existence. This is what Model#has do anyway.

EDIT:

BTW @user.has('name') ? @user.get('name') : 'Anonymous' is not what you expect it to be 😄 CoffeeScript does not have the ternary operator you are used to.

[shiloa]

@gsamokovarov I agree. But this, in my mind, makes for cleaner syntax on that particular method. I was surprised this wasn't already implemented 😉.

I think using something like this will reduce surprises in the long run for several cases, such as:

• when a field isn't present or not yet implemented (stubbed out for design, templating, etc).
• when a field isn't enforced or had it's validation removed.

In the grand scheme of things, it's not a killer feature but rather syntactic sugar that I find quite convenient.

Also, thanks for the EDIT @gsamokovarov - I've updated my pull request's example with yours.

[gsamokovarov]

For the cases you specify I think you can go better with using Model#default. I can see your point, if you are coming from Python (I for one am), but its better to follow JS/CS principles here.

The dict.get(key, default) has the nasty disadvantage that it actually evaluates default all the time. While constructs like obj.key || default can evaluate it only when needed. I actually find myself using dict.get(key, default) less and less even in Python.

[shiloa]

@gsamokovarov I guess this is true if performance is of great importance. If one is performing a lot of model.get(field, fallback) in the page, then it might degrade some.

However, I did give some thought to short-circuiting the evaluation of the fallback var - it's only called when the original attribute isn't present (not including the stack allocation for the args to the function).

I'm no JS expert - certainly in malloc/GC matters - so I'm sure a better implementation could be considered.

[braddunbar]

Mornin' @shiloa! Thanks for the patch. There is some precedent in #1184 and #1634 and I think the reasons there are still valid. While parity between different languages is nice, I think that using model.get(...) || '...' is a simpler, more idiomatic way to accomplish this.

[shiloa]

Hmm, missed those. Thanks anyway.