Ruby - how to join output from a Rails command? - ruby-on-rails

Question

There are multiple calculations I'd like to be able to perform on my database, all centred around a user id, but I don't know how to recombine them after.
Let's say I have three tables for User, Purchases, and DiscountUsage. I want to find the last date a user made a purchase and the total number of discounts used. I would run two separate commands:
User.joins(:purchases).group("users.id").select("users.id", "MAX(purchases.purchase_date)")
User.joins(:discount_usages).group("users.id").select("users.id", "COUNT(*)")
I want my final output to be one table though, joined on users.id, but the output from select isn't the right data type to work on with Rails functions. How can I represent that the users.id values from both calls are the same and thus join them based on those columns?

Answer 1

I assume a User may not have any purchases and not all purchases use discount codes.
However, you want a full listing of each user with their last purchase date and total discount usages over all purchases. You may need to use a right join.
Something like:
query = User.select('users.id AS user_id', 'MAX(purchases.purchase_date) AS last_purchase_date', 'COUNT(discount_usages.id) AS total_discount_usages')
.joins('LEFT JOIN purchases ON purchases.user_id = users.id')
.joins('LEFT JOIN discount_usages ON discount_usages.user_id = purchases.user_id')
.group('users.id')
.to_sql
Then in order to grab the fields you could use:
rows = ApplicationRecord.connection.select_all(query).to_hash
https://guides.rubyonrails.org/active_record_querying.html#select-all
This will give you an array of hashes with keys: 'user_id', 'last_purchase_date', 'total_discount_usages'.
...
<% rows.each do |row| %>
<% row.symbolize_keys! %>
<tr>
<td><%= row[:user_id] %></td>
<td><%= row[:last_purchase_date] %></td>
<td><%= row[:total_discount_usages] %></td>
<tr>
...

Answer 2

You can select aggregates from joined tables and access them in the model by using aliases:
#users = User.joins(:purchases, :discount_usages)
.select(
"users.id",
"MAX(purchases.purchase_date) AS latest_purchase",
"COUNT(discount_usages.*) AS discount_usages_count"
)
.group("users.id")
If you want to hydrate the other attributes of the records select users.* instead of just users.id.
<table>
<thead>
<tr>
<th>Id</th>
<th>Latest Purchase</th>
<th>Discount usages</th>
</tr>
</thead>
<tbody>
<% #users.each do |user| %>
<tr>
<td><%= user.id %></td>
<td><%= user.latest_purchase %></td>
<td><%= user.discount_usages_count %></td>
</tr>
<% end %>
</tbody>
</table>
I want my final output to be one table though, joined on users.id, but
the output from select isn't the right data type to work on with Rails
functions.
Not sure quite what you mean here. This example will return a collection of user records just like a normal query.
If you want a "raw" result with just an array of arrays use .pluck instead of .select.