In an ERB view, you can call
<%= render #cart.line_items %>
and, assuming you have a partial named _line_item.html.erb in the line_items directory, Rails will take care of the rest.
But I'm having some trouble converting this to something similarly succinct in Haml.
I can get something that works by explicitly calling .each on #cart.line_items or an equivalent local variable in the partial, but calling .each is exactly what I'm trying to avoid.
Here's the partial:
%tr
%td= item.product.title
%td= item.quantity
%td.item_price= number_to_currency(item.total_price)
From what I've read so far, this looks like it should produce the same behavior:
= render 'line_items/line_item', collection: #cart.line_items, as: :item
But it's still not working as expected. item isn't being passed as a local:
undefined local variable or method `item' for #<#<Class:0x007fb3c531aee0>:0x007fb3c3692160>
Does Haml have a comparably succinct way to render collections (relative to ERB)? If so, any thoughts as to what I might be missing? Or if there's a better way to do this altogether?
Thanks.
Try this(mention partial explicitly)
= render :partial => 'line_items/line_item', collection: #cart.line_items, as: :item
The render method and how it handles partials is part of Rails, and should work the same with Erb and Haml.
When using render to automatically render a collection, as in this case with render #cart.line_items, Rails will use the name of the class each entry of the collection to determine both the partial to use and the name of the local variable used in that partial.
In your Erb example Rails is using the partial _line_item.html.erb, which suggests that the objects are of type LineItem and so Rails will create a local named line_item. However in your Haml partial you are using the name item, which isn’t being defined hence the error.
Simply change all occurances of item to line_item in your partial, and you will be able to use
= render #cart.line_items
in your Haml, the same as in Erb.
Related
Per the information box on https://guides.rubyonrails.org/layouts_and_rendering.html#using-partials:
Rails also makes a counter variable available within a partial called
by the collection, named after the title of the partial followed by
_counter. For example, when rendering a collection #products the partial _product.html.erb can access the variable product_counter
which indexes the number of times it has been rendered within the
enclosing view.
However, I am getting and error when referencing the counter in my partial. Here is the parent view:
<%= render partial: 'comments/comment_template', collection: #post.comments, as: :c %>
Here is the relevant part of _comment_template.html.erb:
<%= comment_template_counter %>
And here is the error:
undefined local variable or method `comment_template_counter' for #<#<Class: [etc.]
What am I missing?
I believe the documentation is incorrect. As pointed out by pedroadame at https://coderwall.com/p/t0no0g/render-partial-with-collection-has-hidden-index, when using the :as option, I need to use the name of the variable rather than the name of the partial.
Furthermore, if elsewhere in my app I am only rendering the partial once instead of as a collection, I need to sidestep the (same) error message by checking whether the counter is defined.
So in my partial, this now works:
<%= c_counter if defined? c_counter %>
I have a partial: views/nodes/_node.html.erb, which I render on any view like so:
<%= render #nodes %>
When I render it like that, per the Rails Guides, I can use node_counter within the partial as a counter to keep track of various elements within the partial (or say assign an ID that increments every time the partial is rendered).
Now I want to use this same partial, but not rendered like that but rather like this:
<%= render partial: "nodes/node", locals: {node: event.eventable} %>
When I do that, I get this error:
undefined local variable or method `node_counter' for #<#<Class:0x007fa3b6ca9778>:0x007fa3b58a60c8>
That is because I never specified/passed node_counter as a local variable.
So how do I specify node_counter to start at 0 and always increment every time this partial is rendered - if I can't do it the Rails Way?
For reference, the Rails Guide says this:
Rails also makes a counter variable available within a partial called by the collection, named after the member of the collection followed by _counter. For example, if you're rendering #products, within the partial you can refer to product_counter to tell you how many times the partial has been rendered. This does not work in conjunction with the as: :value option.
In your calling view, you can use each_with_index:
<% #foo.each_with_index do |foo, index| %>
<%= render 'nodes/node', f:foo, node_counter:index %>
Then in your _nodes/node partial, node_counter will increment starting at 0.
The reason that node_counter is not available for you in your second call is that you are calling the partial with a member, not a collection.
I was going through a video and where he was rendering something like below example:
<div>
<%= render posts %> # no quotes to posts
</div>
Though he has even created a partial with _posts.html.erb, he is calling with quotes to posts.
Though he has mentioned something about it like it calls, active record model, then class and then something...i could not understand it properly. Can anyone explain clearly this with simple example.
Render with quotes
<%=render 'post'%>
Rails is going to look in the current folder for a partial file which starts with _
Render without quotes
Is still going to use the same partial, but post in this case is a variable. I think this is translating to this:
<%= render partial: "post", locals: {any_string: your_variable(in this case is post)} %>
Again I haven't checked that.
The _post.html.erb is the partial, which can look like this:
<b><%=any_string%></b>
If your_variable which was assigned to any_string will contain the string 'My name is'
Your partial will print 'My name is' in bold.
Anyway partial are more complex, and they are used for DRY-ing (Don't repeat yourself) the code.
You can see more examples here.
With quotes then you are explicitly rendering a partial of that name. Without quotes something quite interesting is happening. posts (without quotes) is a variable that will be an activemodel list of records.
Now what the call to render does is it will look at the type of each of the models and then find the correct partial for the model (which will be the name of the model camel_cased) and render each one in turn.
EDIT:
If you have a model called Post and you assign some of those records to a variable (he uses posts I assume but I'll use foo to disambiguate) like so:
foo = Post.all
then by calling render foo the render function will see that you have an activerecord collection of records, it will then check the model associated with these records (Post in our example) and will loop through all of them rendering them to a partial called _post.html.erb with a local variable for each record assigning the record to post.
<%= render foo %>
is equivalent to:
<% foo.each do |my_post| %>
<%= render partial: "post", locals: {post: my_post} %>
<% end %>
I'm trying to pass in the variable "post" to a partial. The partial is being used on both my show#view & I'm also rendering a collection using it. Here's what it looks like (notice the "#"):
##Show#View
<%= render 'my_partial/my_view', post: #post %>
##Collection ## (I'm not using the "#" symbol)
<%= render 'my_partial/my_view', post: post %>
#Mypartial
<% if #post.something? %>
## do this
<% else %>
## do that
<% end %>
And then I get the beautiful NoMethodError undefined methodsomething?' for nil:NilClass` page when using it in my collection. I know why I'm getting it, I'm just wondering what's the DRY way(s) of getting this to work? Should I just create another partial?
Thank you
Gave my solution below.. Though, it's probably not the best way...
Why are you referencing #post in your partial? You should use post instead, that is the entire point of what you are doing (passing variables to a partial as local variables).
You must change the rendering to:
<%= render 'my_partial/my_view', locals: { post: #post } %>
Take a look in the rails guide: http://guides.rubyonrails.org/layouts_and_rendering.html
Search for "3.4.4 Passing Local Variables" and you will find more information about this whole stuff.
And for fixing your collection problem also check out the rails guide and search for: "3.4.5 Rendering Collections".
Ryan Bates' nifty_scaffolding, for example, does this
edit.html.erb
<%= render :partial => 'form' %>
new.html.erb
<%= render :partial => 'form' %>
_form.html.erb
<%= form_for #some_object_defined_in_action %>
That hidden state makes me feel uncomfortable, so I usually like to do this
edit.html.erb
<%= render :partial => 'form', :locals => { :object => #my_object } %>
_form.html.erb
<%= form_for object %>
So which is better: a) having partials access instance variables or b) passing a partial all the variables it needs?
I've been opting for b) as of late, but I did run into a little pickle:
some_action.html.erb
<% #dad.sons.each do |a_son| %>
<%= render :partial => 'partial', :locals => { :son => a_son } %>
<% end %>
_partial.html.erb
The son's name is <%= son.name %>
The dad's name is <%= son.dad.name %>
son.dad makes a database call to fetch the dad! So I would either have to access #dad, which would be going back to a) having partials access instance variables or I would have to pass #dad in locals, changing render :partial to
<%= render :partial => 'partial', :locals => { :dad => #dad, :son => a_son } %>, and for some reason passing a bunch of vars to my partial makes me feel uncomfortable. Maybe others feel this way as well.
Hopefully that made some sense. Looking for some insight into this whole thing... Thanks!
In recent versions of Rails it is quite a bit easier to render partials and pass locals to them. Instead of this.
<%= render :partial => 'form', :locals => { :item => #item } %>
You can do this.
<%= render 'form', :item => #item %>
I don't do this in the Nifty Scaffold generator to keep backwards compatibility, but I'll change this in a future release.
As for whether it's acceptable to use instance variables in partials. I think it is. In all practicality, what is the downside? Certainly things can get out of hand if you aren't consistent, but I like to apply these guidelines.
Never create an instance variable just to share it between partials. Usually this means you will only be sharing the controller resource object.
If the partial is the same name as the resource, pass it as a local with <%= render #item %>.
If the partial will be shared across multiple controllers then only use locals.
This is what works well for me anyway.
Bonus tip: if you find yourself passing in a lot of locals into a partial and you want some of them to be optional, create a helper method which renders the partial. Then always go through the helper method so you can make a clean interface with optional args for rendering the partial.
Using #instance_variables in partials is bad design.
Using instance variable in partials works, but it can make it harder to maintain applications if changes are ever needed.
The downside of using instance variables in partials is that you create a dependency in the partial to something outside the partial's scope (coupling). This makes the partial harder to reuse, and can force changes in several parts of the application when you want to make a change in one part.
Partials that use instance variables:
must change when the instance variable in any controller that uses the partial changes either the instance variable name or its type or data structure
cause all controller actions that use the partial to change in the same way at the same time when there are changes to how the instance variable is used
discourage reuse, as they can only easily be reused in actions that set up instance variables with the same name and data
Instead, pass locals to the partials:
<%= render 'reusable_partial', :item => #item %>
Now, because the partial only references item and not #item, the action that renders the view that renders the reusable_partial is free to change without affecting the reusable_partial and the other actions/views that render it:
<%= render 'reusable_partial', :item => #other_object.item %>
Also, this can be reused in contexts where there is no #item:
<%= render 'reusable_partial', :item => #duck %>
If my #duck changes in the future and no longer quacks like reusable_partial expects it to (the object's interface changes), I can also use an adapter to pass in the kind of item that reusable_partial expects:
<%= render 'reusable_partial', :item => itemlike_duck(#duck) %>
Always?
There are plenty of situations where you probably don't need de-coupled partials like this, and it's easier in the short run to use an instance variable. However, it's hard to predict the future needs of your application.
As such, this makes for good general practice while having relatively low cost.
You can have it both ways. At the top of your partial:
<% item ||= #item %>
That way, it works with or without passing the local variable, providing a sane default, but not inhibiting alternate usage of the partial.
I vote for a) for a very specific reason -- DRY! If you start passing a variable like that, the next thing you know it's a mess. Let's say you need to change the way your variable is named or something else about it. You'll need to go to ALL your views and change them instead of ONE partial.
Also, if you change your partial it will change on all your views, so you'll need to know which views are used. A proper IDE should be able to help you with that, but I also like having a small comment section at the top of the view where I just mention where it's used and why. This helps another programmer and it helps you to remember in case you need to come back to a partial and modify. But the whole point of the partial is to call it WITHOUT having to pass anything from the view, so that you don't have to modify all places where partial is called from if that variable changes somehow.
Ultimately this is a design choice, and to be honest unless you are running a facebook the extra lookup you do is not that big of a deal, but it's just not very DRY.
P.S.: Just thought about it. You can actually abstract the way you call partial in a helper method, so then if the way you call your partial needs to change, you just need to modify one place.