Complex DataMapper query association

Question

im a beginner with DataMapper ORM, so i have question about complex querying.

First, here is simplified data objects:

class User  
    property :id, Serial
    property :login, String

    has n, :actions
end

class Item
    property :id, Serial
    property :title

    has n, :actions
  has n, :users, :through => :actions
end

class Action
    property :user_id, Integer
    property :item_id, Integer

    belongs_to :item
    belongs_to :user
end

This is how data in db looks like:

+ ------- + + ------- + + ------- +
| Users   | | Items   | | Actions |
+ ------- + + ------- + + ------- +
| 1  | u1 | | 3  | i1 | | 1  | 4  |
| 2  | u2 | | 4  | i2 | | 1  | 3  |
| ....... | | 5  | i3 | | 1  | 4  |
+ ------- + | ....... | | 1  | 5  |
            + ------- + | 1  | 6  |
                        | 1  | 3  |
                        | ....... |
                        + ------- +

So, for example User 1 has viewed some items N time. And what i cant figure out, how to select items and their action amount relating to user.

For example, the result for user 1 should be like this:

+ -------------------- |
| Items (item_id, num) |
+ -------------------- |
| 3, 2                 |
| 4, 2                 |
| 5, 1                 |
| 6, 1                 |
+ -------------------- +

P.S. regular SQL query that matches my needs:

SELECT i.id, i.title, COUNT(*) as 'num'
FROM actions a
JOIN items i on i.id = a.item_id
WHERE a.user_id = {USERID}
GROUP by a.id
ORDER BY num DESC
LIMIT 10;

So, how to do this and is there are any docs about complex datamapper queries?

Answer 1

In case anybody is still wondering:

Action.aggregate(:item_id, :all.count, :user_id => 1, :order => [item_id.asc])

Will return something like

[ [ 3, 2 ],
  [ 4, 2 ],
  [ 5, 1 ],
  [ 6, 1 ]
]

No way to order by all.count here, but it gets you the data you want :)

Answer 2

As far as I know, there isn't a group by operator in datamapper or any of its plugins. If there were, it would go in dm-aggregates along with the aggregation functions(count,min,max,avg). That makes it difficult to replicate what you want in one query without using sql.

You could try something like this:

require 'dm-aggregates'

Item.all.map do |item|
  [item.title,item.actions.count(:user_id=>@user_id)]
end

But, you could just as easily take your sql and wrap it in a fn.

class User
  def item_views
  repository.adapter.query "SELECT i.id, i.title, COUNT(*) as 'num'
    FROM actions a
    JOIN items i on i.id = a.item_id
    WHERE a.user_id = {USERID}
    GROUP by a.id
    ORDER BY num DESC
    LIMIT 10;"
  end
end

repository.adapter.query returns an Array of structs so you could do things like

user.item_views[0].title