"merging" multiple models. To create a "recent activity" box

Question

How do you merge models so that I can have the last 10 Posts, Feed Entries and Private Messages displayed in order?

Posts are stored in the "Post" model and ordered on "created_at"

Feed Entries are stored in "Planet" and ordered on "published_at"

Private Messages are stored in "Message" and need to be filtered with:

:conditions => "receiver_id = #{current_user.id}"

and ordered on "created_at"

Answer 1

You have to:

1. query elements for each model
2. merge them in a common format
3. sort and limit

Here is some code:

class Activity < Struct.new(:title, :text, :date); end

limit = 10
activities = []
activities += Post.all(:order => 'created_at DESC', :limit => limit).map do |post|
  Activity.new(post.title, post.summary, post.created_at)
end

activities += Planet.all(:order => 'published_at DESC', :limit => limit).map do |planet|
  Activity.new(planet.title, planet.message, planet.published_at)
end

activities += Message.all(:conditions => ['receiver_id = ?', current_user.id], :order => 'created_at DESC', :limit => limit).map do |message|
  Activity.new(message.title, message.text, message.created_at)
end

# descending sort by 'date' field
sorted_activities = activities.sort_by(&:date).reverse

# 10 most recent elements across all models
@activities = sorted_activities[0..(limit-1)]

Of course, depending on your models, you will have to change which method is used as "title" or "text".

But if you happen to need many of such idioms, you should use Single Table Inheritance as we do in zena (a rails CMS).