Retrieve previous and next members in a work

app/controllers/admin/assets_controller.rb

@@ -30,6 +30,9 @@ def show
 
     @exiftool_result = Kithe::ExiftoolCharacterization.presenter_for(@asset&.exiftool_result)
 
+    getter = MemberPreviousAndNextGetter.new(@asset)
+    @previous_url, @next_url = getter.previous_url, getter.next_url
+
     if @asset.stored?
       @checks = @asset.fixity_checks.order('created_at asc')
       @latest_check   = @checks.last

app/services/member_previous_and_next_getter.rb

@@ -0,0 +1,53 @@
+# Finds the next and previous members in a work.
+# In cases where positions are noncontiguous, non-unique, or nil,
+# you can still navigate through the members sequentially.
+class MemberPreviousAndNextGetter
+  # @param member [Kithe::Model]
+  def initialize(member)
+    @member = member
+  end
+
+  def next_url
+    @next_url ||= url(:next)
+  end
+
+  def previous_url
+    @previous_url ||= url(:previous)
+  end
+
+  private
+
+  def url(previous_or_next)
+    return nil unless @member.parent.present?
+    if (neighbor_id = query["#{previous_or_next}_friendlier_id"])
+      # ok, we have a neighbor. construct the right kind of link to it.
+      neighbor_type = query["#{previous_or_next}_type"]
+      if neighbor_type == 'Work'
+        Rails.application.routes.url_helpers.admin_work_path(neighbor_id)
+      elsif neighbor_type == 'Asset'
+        Rails.application.routes.url_helpers.admin_asset_path(neighbor_id)
+      end
+    end
+  end
+
+  # Takes less than a millisecond.
+  def query
+    @query ||= ActiveRecord::Base.connection.execute("""
+      SELECT * FROM (
+        SELECT
+          id,
+          LAG  (friendlier_id, 1) over (order by position, id) AS previous_friendlier_id,
+          LAG  (type, 1)          over (order by position, id) AS previous_type,
+          LEAD (friendlier_id,1)  over (order by position, id) AS next_friendlier_id,
+          LEAD (type,1)           over (order by position, id) AS next_type
+          FROM (
+            SELECT id, friendlier_id, type, position
+            FROM kithe_models
+            WHERE parent_id = '#{@member.parent.id}'
+        ) AS members
+      ) AS all_members
+      WHERE id = '#{@member.id}';
+    """.squish)[0]
+  end
+end
+

app/views/admin/assets/_prev_next.html.erb

@@ -0,0 +1,17 @@
+<p>
+
+  <% if @previous_url %>
+    <%= link_to "« Previous", @previous_url %>
+  <% else %>
+    <%= link_to "« Previous", "", class: "btn-link disabled" %>
+  <% end %>
+
+|
+
+  <% if @next_url %>
+    <%= link_to "Next »", @next_url %>
+  <% else %>
+    <%= link_to "Next »", "", class: "btn-link disabled" %>
+  <% end %>
+
+</p>

app/views/admin/assets/show.html.erb

@@ -11,6 +11,7 @@
     <p class="text-danger">NO PARENT</p>
 <% end %>
 
+<%= render 'admin/assets/prev_next' %>
 
 <p>
   <%= link_to "Edit", edit_path(@asset), class: "btn btn-primary #{"disabled" unless can?(:update, @asset)}"%>

spec/services/member_previous_and_next_getter_spec.rb

@@ -0,0 +1,80 @@
+require 'rails_helper'
+
+describe MemberPreviousAndNextGetter, type: :model do
+  let(:members) { parent_work.members.order(:position, :id) }
+
+  let(:actual_urls) do
+    members.map do |m|
+      getter = MemberPreviousAndNextGetter.new(m)
+      [ getter&.previous_url, getter&.next_url ]
+    end  
+  end
+
+  let(:wrk_prefix) { "/admin/works/" }
+  let(:ast_prefix) { "/admin/asset_files/" }
+
+  describe "navigation between members of a parent work" do
+    context "four assets and one child work" do
+      let(:parent_work) { FactoryBot.create(:work, :with_assets, asset_count: 4) }
+      let!(:child_work) { FactoryBot.create(:work, :with_assets, parent: parent_work, position: 5) }
+      it "finds previous and next members correctly" do
+        expect(actual_urls).to eq ([
+          [ nil,
+            "#{ast_prefix}#{members[1].friendlier_id}"],
+
+          [ "#{ast_prefix}#{members[0].friendlier_id}",
+            "#{ast_prefix}#{members[2].friendlier_id}"],
+
+          [ "#{ast_prefix}#{members[1].friendlier_id}",
+            "#{ast_prefix}#{members[3].friendlier_id}"],
+
+          [ "#{ast_prefix}#{members[2].friendlier_id}",
+            "#{wrk_prefix}#{members[4].friendlier_id}"],
+
+          [ "#{ast_prefix}#{members[3].friendlier_id}",
+            nil]
+        ])
+      end
+    end
+
+    context "non-consecutive, duplicate and nil positions" do
+      let (:basic_uuid) {'00000000-0000-4000-8000-%012x'}
+      let!(:parent_work) { FactoryBot.create(:work, members: [
+        FactoryBot.create(:asset, id: basic_uuid % 1, position: nil),
+        FactoryBot.create(:asset, id: basic_uuid % 2, position: 7  ),
+        FactoryBot.create(:asset, id: basic_uuid % 3, position: 29 ),
+        FactoryBot.create(:work,  id: basic_uuid % 4, position: nil),
+        FactoryBot.create(:work,  id: basic_uuid % 5, position: 7  )
+        ] )
+      }
+      it "finds previous and next members correctly" do
+        expect(actual_urls).to eq ([
+          [ nil,
+            "#{wrk_prefix}#{members[1].friendlier_id}"],
+
+          [ "#{ast_prefix}#{members[0].friendlier_id}",
+            "#{ast_prefix}#{members[2].friendlier_id}"],
+
+          [ "#{wrk_prefix}#{members[1].friendlier_id}",
+            "#{ast_prefix}#{members[3].friendlier_id}"],
+
+          [ "#{ast_prefix}#{members[2].friendlier_id}",
+            "#{wrk_prefix}#{members[4].friendlier_id}"],
+
+          [ "#{ast_prefix}#{members[3].friendlier_id}",
+            nil]
+        ])
+      end
+    end
+
+    context "parent_work is absent" do
+      let(:members) {[
+        FactoryBot.create(:asset),
+        FactoryBot.create(:asset),
+      ]}
+      it "doesn't throw an error" do
+        expect(actual_urls).to eq([[nil, nil], [nil, nil]])
+      end
+    end
+  end
+end