openstreetmap-website | The Rails application that powers | Map library

I forked a Ruby on Rails project from https://github.com/openstreetmap/openstreetmap-website but I'm new in Ruby.

I need to make a list with a column of usernames and another column with the sum of changes made by that user.

As you can see, there is a table named "changesets" in the database:

irb(main):002:0> ActiveRecord::Base.connection.tables

=> ["schema_migrations", "ar_internal_metadata", "current_way_tags", "current_ways", "diary_entries", "friends", "gps_points", "gpx_file_tags", "messages", "gpx_files", "users", "way_tags", "ways", "user_preferences", "user_tokens", "way_nodes", "diary_comments", "current_relations", "current_node_tags", "node_tags", "current_relation_tags", "nodes", "acls", "relation_tags", "current_way_nodes", "current_nodes", "current_relation_members", "changesets", "relation_members", "changeset_tags", "languages", "relations", "oauth_nonces", "oauth_tokens", "user_roles", "notes", "user_blocks", "client_applications", "note_comments", "redactions", "changesets_subscribers", "changeset_comments", "reports", "issues", "issue_comments", "diary_entry_subscriptions", "delayed_jobs"]

And the changesets table have the following columns:

irb(main):007:0> Changeset.column_names

=> ["id", "user_id", "created_at", "min_lat", "max_lat", "min_lon", "max_lon", "closed_at", "num_changes"]

And the users table have the following columns:

irb(main):008:0> User.column_names

=> ["email", "id", "pass_crypt", "creation_time", "display_name", "data_public", "description", "home_lat", "home_lon", "home_zoom", "nearby", "pass_salt", "image_file_name", "email_valid", "new_email", "creation_ip", "languages", "status", "terms_agreed", "consider_pd", "auth_uid", "preferred_editor", "terms_seen", "description_format", "image_fingerprint", "changesets_count", "traces_count", "diary_entries_count", "image_use_gravatar", "image_content_type", "auth_provider", "home_tile", "tou_agreed", "ranked"]

An user can have several changesets, but each changeset belongs to only one user. So the second column of my list should be the sum of the num_changes of all the changesets of that user.

How could I display that information? I think it would be something like this: