Rails 4 - passing variable to partial

From the Rails api on PartialRender:

Rendering the default case

If you're not going to be using any of the options like collections or layouts, you can also use the short-hand defaults of render to render partials.

Examples:

# Instead of <%= render partial: "account" %>
<%= render "account" %>

# Instead of <%= render partial: "account", locals: { account: @buyer } %>
<%= render "account", account: @buyer %>

# @account.to_partial_path returns 'accounts/account', so it can be used to replace:
# <%= render partial: "accounts/account", locals: { account: @account} %>
<%= render @account %>

# @posts is an array of Post instances, so every post record returns 'posts/post' on `to_partial_path`,
# that's why we can replace:
# <%= render partial: "posts/post", collection: @posts %>
<%= render @posts %>

So, you can use pass a local variable size to render as follows:

<%= render @users, size: 50 %>

and then use it in the _user.html.erb partial:

<li>
    <%= gravatar_for user, size: size %>
    <%= link_to user.name, user %>
</li>

Note that size: size is equivalent to :size => size.

You need the full render partial syntax if you are passing locals

<%= render @users, :locals => {:size => 30} %>

Becomes

<%= render :partial => 'users', :collection => @users, :locals => {:size => 30} %>

Or to use the new hash syntax

<%= render partial: 'users', collection: @users, locals: {size: 30} %>

Which I think is much more readable

Either

render partial: 'user', locals: {size: 30}

Or

render 'user', size: 30

To use locals, you need partial. Without the partial argument, you can just list variables directly (not within locals)

Don't use locals in Rails 4.2+

In Rails 4.2 I had to remove the locals part and just use size: 30 instead. Otherwise, it wouldn't pass the local variable correctly.

For example, use this:

<%= render @users, size: 30 %>

If you are using JavaScript to render then use escape_JavaScript("<%=render partial: partial_name, locals=>{@newval=>@oldval}%>");