Currently I am using fragment caching with an object key. And I expire the cache using Touch with updates the updated_at column and then the current cache become obsolete. And a new cache is generated the next time.
<% cache do product %>
The cache key for an object looks like this
table_name/ID-updated_at
Now I want to cache my products somewhere else. To do so I would like to do same thing as before which is simply give the object in cache options, but I can't do that :(
It would be great if I could generate a key like this in order to still have my cache expired with the Touch.
table_name/ID-something_updated_at
<% cache product,something do %>
Unfortunately this is not possible, does someone has any idea how I could solve my problem?
Greg
You can pass any old array you like to the fragment, so it's a pretty simple change to achieve what you're after:
<% cache [product, other, product.updated_at] do %>
...
<% end %>
Related
I use fragment caching in Rails 4.x.
I specify my own key using something like this in my view:
<% cache "my-key" %>
html
<% end %>
This produces a key similar to this in my Redis database:
views/my-key/dca9add42d461d4c76103c08d12a6571
When I want to expire this key, I use the expire_fragment command in a controller like so:
expire_fragment("my-key")
But this command only produces a cache key like so:
views/my-key
What I don't understand is, what is the /dca9add42d461d4c76103c08d12a6571 part?
How does the original cache command not produce it, or how can I reproduce the /dca9add42d461d4c76103c08d12a6571 suffix when I want to expire the fragment.
I'm not sure if this is by design, or whether there is a bug in Rails. But what is happening is the suffix /dca9add42d461d4c76103c08d12a6571 that's added to the end of the cache key when I use the cache command in my views, is a digest, which you can skip by calling cache like so:
<% cache("my-key", { :skip_digest: true }) %>
html
<% end %>
That way the suffix (digest) gets left off the key.
Now where I think a possible bug (?) is coming in, is that the expire_fragment command doesn't add the digest to the end of the key - so I never actually end up expiring my fragments.
So my answer for now - is to use :skip_digest: true if I want to manually expire them.
According to the Rails documentation, you can just pass in the key as a string:
expire_fragment('my-key')
http://guides.rubyonrails.org/caching_with_rails.html#fragment-caching
If you're doing it from the command line, you might have to go through whatever controller the view is rendered by:
MyController.new.expire_fragment 'my-key'
I have a block
<% cache 'unique_key', 60.minutes.from_now do %>
...
<% begin %>
...
<% rescue %>
...
<%end>
<% end %>
and I'm trying to make the implementation more robust by only caching (and thus allowing the user to see) the rescue message if there isn't a previous value already in the cache. Currently, if the response in the begin block sends back an error for any reason, I'm caching the user viewed error message. I would prefer to fall back onto the old cached data. The problem that I can't get past is -
Where is cache storing the data?
Every time I try Rails.cache.read 'unique_key', I get nil back. Is cache not storing the value in memcached? Is there a way that I can dump the cache to screen?
I couldn't follow the rails source. It seemed to me the the fragment_for method in cache was a rails 3 thing, and thus, I didn't debug further.
The cache view helper constructs a cache key based on the arguments you give it. At a minimum it adds the prefix 'views/' to the key.
You can use the fragment_cache_key helper to find out what cache key rails is using for any of your calls to cache. If you just want to grab what is currently stored, read_fragment does that. Of course with your particular usage, if your block is executed again it is because the 60 minutes are up: the cached value has been deleted from memcache.
With the memcache store you can't list all of the keys currently in the store - it's just something thy memcached itself doesn't support.
I solved this by using the fetch method. I used
<% Rails.cache.fetch('unique_key', :expires_in => 60.minutes){
begin
...
rescue
...
end
} %>
When I did this, I could successfully find the key. I'm still not sure why I couldn't find the cached data after adding the fragment_cache_key that I found, but using Rails.cache.fetch seemed to do the trick.
I am running into an issue when looking into fragment-level caching within my Rails 3.0.4 application with memcached. I am a bit confused with what is going on, but I think it's something to do with the way the output is being pulled from within the caching region. I am running memcached locally in -vv mode, and can see the key for the fragment getting saved/pulled correctly, the problem is the value of the item within memcached.
Here is what I'm doing:
< ... html before ... >
<%= cache("item_#{i.id}") do %>
<%= render :partial => 'shared/item', :locals => { :item => i, :functionality => [:set_as_default] } %>
<% end %>
< ... html after ... >
When I look at the value of the key within the cache, it has html from within the page that is in that fragment cache block, but ALSO OUTSIDE of that (from both the html before and html after areas). Here is the interesting part though, and is kind of the reason I think its related to capturing the output--it doesn't do the whole page, only some of the html before and some after.
According to the rails fragment cacheing guide, I think I'm doing things correctly (http://guides.rubyonrails.org/caching_with_rails.html#fragment-caching). Does anyone have thoughts as to what could be going on?
Your help is much appreciated!
-Eric
In this case, you are using ERB incorrectly. Basically take out the = sign. What your doing is your returning the value of the block too and hence why you are seeing double output.
<% cache("item_#{i.id}") do %>
Also, ActiveRecord objects respond to an internally baked in #cache_key method. Try to take advantage of that. The default #cache_key for an ActiveRecord object uses the class name, the object id and the updated_at timestamp too. The cache method should be able to take multiple args or an array and it will inturn call cache_key for every object that responds to it. Using this method, it means you will cache miss when the object is updated to, pretty cool stuff. So, IIRC
<% cache("item",i) do %>
I am implementing caching into my Rails project via Memcached and particularly trying to cache side column blocks (most recent photos, blogs, etc), and currently I have them expiring the cache every 15 minutes or so. Which works, but if I can do it more up-to-date like whenever new content is added, updated or whatnot, that would be better.
I was watching the episode of the Scaling Rails screencasts on Memcached http://content.newrelic.com/railslab/videos/08-ScalingRails-Memcached-fixed.mp4, and at 8:27 in the video, Gregg Pollack talks about intelligent caching in Memcached in a way where intelligent keys (in this example, the updated_at timestamp) are used to replace previously cached items without having to expire the cache. So whenever the timestamp is updated, the cache would refresh as it seeks a new timestamp, I would presume.
I am using my "Recent Photos" sideblock for this example, and this is how it's set up...
_side-column.html.erb:
<div id="photos"">
<p class="header">Photos</p>
<%= render :partial => 'shared/photos', :collection => #recent_photos %>
</div>
_photos.html.erb
<% cache(photos) do %>
<div class="row">
<%= image_tag photos.thumbnail.url(:thumb) %>
<h3><%= link_to photos.title, photos %></h3>
<p><%= photos.photos_count %> Photos</p>
</div>
</div>
<% end %>
On the first run, Memcached caches the block as views/photos/1-20110308040600 and will reload that cached fragment when the page is refreshed, so far so good. Then I add an additional photo to that particular row in the backend and reload, but the photo count is not updated. The log shows that it's still loading from views/photos/1-20110308040600 and not grabbing an updated timestamp. Everything I'm doing appears to be the same as what the video is doing, what am I doing wrong above?
In addition, there is a part two to this question. As you see in the partial above, #recent_photos query is called for the collection (out of a module in my lib folder). However, I noticed that even when the block is cached, this SELECT query is still being called. I attempted to wrap the entire partial in a block at first as <% cache(#recent_photos) do %>, but obviously this doesn't work - especially as there is no real timestamp on the whole collection, just it's individual items of course. How can I prevent this query from being made if the results are cached already?
UPDATE
In reference to the second question, I found that unless Rails.cache.exist? may just be my ticket, but what's tricky is the wildcard nature of using the timestamp...
UPDATE 2
Disregard my first question entirely, I figured out exactly why the cache wasn't refreshing. That's because the updated_at field wasn't being updated. Reason for that is that I was adding/deleting an item that is a nested resource in a parent, and I probably need to implement a "touch" on that in order to update the updated_at field in the parent.
But my second question still stands...the main #recent_photos query is still being called even if the fragment is cached...is there a way using cache.exists? to target a cache that is named something like /views/photos/1-2011random ?
One of the major flaws with Rails caching is that you cannot reliably separate the controller and the view for cached components. The only solution I've found is to embed the query in the cached block directly, but preferably through a helper method.
For instance, you probably have something like this:
class PhotosController < ApplicationController
def index
# ...
#recent_photos = Photos.where(...).all
# ...
end
end
The first instinct would be to only run that query if it will be required by the view, such as testing for the presence of the cached content. Unfortunately there is a small chance that the content will expire in the interval between you testing for it being cached and actually rendering the page, something that will lead to a template rendering error when the nil-value #recent_photos is used.
Here's a simpler approach:
<%= render :partial => 'shared/photos', :collection => recent_photos %>
Instead of using an instance variable, use a helper method. Define your helper method as you would've the load inside the controller:
module PhotosHelper
def recent_photos
#recent_photos ||= Photos.where(...).all
end
end
In this case the value is saved so that multiple calls to the same helper method only triggers the query once. This may not be necessary in your application and can be omitted. All the method is obligated to do is return a list of "recent photos", after all.
A lot of this mess could be eliminated if Rails supported sub-controllers with their own associated views, which is a variation on the pattern employed here.
As I've been working further with caching since asking this question, I think I'm starting to understand exactly the value of this kind of caching technique.
For example, I have an article and through a variety of things I need for the page which include querying other tables, maybe I need to do five-seven different queries per article. However, caching the article in this way reduces all those queries to one.
I am assuming that with this technique, there always needs to have at least "one" query, as there needs to be "some" way to tell whether the timestamp has been updated or not.
I would like to cache my fragment page in my rails application by time.
I found this plugin to do this => ici but any download is available.
I searched in the rails doc but I don't found how to cache my fragment by time.
Are you know another plugin to do this or another method to do this ?
Thanks.
Creating a time-based cache key is quite simple.
Here's an example.
Now in your app you can write
<% cache :expires => CacheKey.expirable(:hour) do %>
...
<% end %>
If you want a more accurate control (for example 5.minutes instead of simply 1 minute), you can easily adapt the module in order to dynamically generate the cache key reading the time value passed as parameter.
An other approach is to check the last-modified time of the cache file. Here's a plugin.