Add organisations fetcher

Add a rake task for scraping the GOV.UK organisations API [[1]] and
saving the data we want to our database. This is based on what the
GOV.UK Signon team do themselves [[2]].

Note, I considered adding the [gds-api-adapters gem][3] for this, but in
the end I found I could implement the functionality we needed easily
enough using the Ruby standard library.

[1]: https://www.api.gov.uk/gds/gov-uk-organisations/#gov-uk-organisations
[2]: https://github.com/alphagov/signon/blob/95bd01cb7902e96b6303a274e9301d83e21a32b7/lib/organisations_fetcher.rb
[3]: https://github.com/alphagov/gds-api-adapters

lib/organisations_fetcher.rb

@@ -0,0 +1,58 @@
+# GOV.UK is the canonical source for organisations, so we need to keep our
+# organisations up-to-date in order to provide accurate information on user
+# membership of organisations.
+#
+# Based on similar code in Signon app:
+# https://github.com/alphagov/signon/blob/b7a53e282c55d8ef3ab6369a7cb358b6ae100d27/lib/organisations_fetcher.rb
+class OrganisationsFetcher
+  def call
+    organisations.each do |organisation_data|
+      update_or_create_organisation(organisation_data)
+    end
+  rescue ActiveRecord::RecordInvalid => e
+    raise "Couldn't save organisation #{e.record.slug} because: #{e.record.errors.full_messages.join(',')}"
+  end
+
+private
+
+  def organisations
+    @organisations ||= get_json_with_subsequent_pages(URI("https://www.gov.uk/api/organisations"))
+  end
+
+  def update_or_create_organisation(organisation_data)
+    govuk_content_id = organisation_data[:details][:content_id]
+    slug = organisation_data[:details][:slug]
+
+    organisation = Organisation.find_by(govuk_content_id:) ||
+      Organisation.find_by(slug:) ||
+      Organisation.new(govuk_content_id:)
+
+    update_data = {
+      govuk_content_id:,
+      slug:,
+      name: organisation_data[:title],
+    }
+
+    organisation.update!(update_data)
+  end
+
+  def get_json_with_subsequent_pages(uri)
+    next_page_uri = uri
+    Enumerator.new do |yielder|
+      while next_page_uri
+        page = get_json(next_page_uri)
+        page[:results].each { |i| yielder << i }
+        next_page_uri = page.key?(:next_page_url) ? URI(page[:next_page_url]) : nil
+      end
+    end
+  end
+
+  def get_json(uri)
+    response = Net::HTTP.get_response(uri)
+    if response.is_a? Net::HTTPSuccess
+      JSON.parse(response.body, symbolize_names: true)
+    else
+      raise "error fetching organisations: #{response.code}: #{response.body}"
+    end
+  end
+end

lib/tasks/organisations.rake

@@ -0,0 +1,8 @@
+require_relative "../organisations_fetcher"
+
+namespace :organisations do
+  desc "Update organisations table using data from GOV.UK"
+  task fetch: :environment do
+    OrganisationsFetcher.new.call
+  end
+end