[Illustrative] WITH RECURSIVE link expansion

app/models/edition.rb

@@ -223,6 +223,10 @@ def web_url
     Plek.website_root + base_path
   end
 
+  def linked_editions
+    Queries::RecursiveLinkExpansion::RecursiveLinkExpansion.new(self).call
+  end
+
 private
 
   def renderable_content?

app/queries/recursive_link_expansion/base_edition.rb

@@ -0,0 +1,25 @@
+module Queries
+  module RecursiveLinkExpansion
+    ##
+    # Selects the columns required for recursive link expansion from the base edition (the root edition in the tree)
+    class BaseEdition
+      def initialize(edition_with_document, links = [])
+        @edition = edition_with_document
+        @links = links
+      end
+
+      def call
+        Edition.with_document.where(id: @edition.id).select(
+          "'0 base' as type",
+          "'{}'::text[] as path",
+          "documents.content_id",
+          "documents.locale",
+          "editions.id as edition_id",
+          "0 as position",
+          "editions.state",
+          ActiveRecord::Base.send(:sanitize_sql_array, ["?::jsonb as links", @links.to_json]),
+        )
+      end
+    end
+  end
+end

app/queries/recursive_link_expansion/forward_edition_links.rb

@@ -0,0 +1,24 @@
+module Queries
+  module RecursiveLinkExpansion
+    class ForwardEditionLinks
+      def call
+        # TODO: - parameterize locale / state
+        Edition
+          .from("lookahead")
+          .joins("INNER JOIN links ON (links.edition_id = lookahead.edition_id AND links.link_type = lookahead.type AND lookahead.reverse = false)")
+          .joins("INNER JOIN documents ON (documents.content_id = links.target_content_id AND documents.locale='en')")
+          .joins("INNER JOIN editions ON (editions.document_id = documents.id AND editions.state='published')")
+          .select(
+            "'1 forward edition link' as type",
+            "path || lookahead.content_id::text || lookahead.type AS path",
+            "documents.content_id",
+            "documents.locale",
+            "editions.id as edition_id",
+            "links.position",
+            "editions.state",
+            "lookahead.links",
+          )
+      end
+    end
+  end
+end

app/queries/recursive_link_expansion/forward_link_set_links.rb

@@ -0,0 +1,24 @@
+module Queries
+  module RecursiveLinkExpansion
+    class ForwardLinkSetLinks
+      def call
+        # TODO: - parameterize locale / state
+        Edition
+          .from("lookahead")
+          .joins("INNER JOIN links ON (links.link_set_content_id = lookahead.content_id AND links.link_type = lookahead.type AND lookahead.reverse = false)")
+          .joins("INNER JOIN documents ON (documents.content_id = links.target_content_id AND documents.locale='en')")
+          .joins("INNER JOIN editions ON (editions.document_id = documents.id AND editions.state='published')")
+          .select(
+            "'2 forward link set link' as type",
+            "path || lookahead.content_id::text || lookahead.type AS path",
+            "documents.content_id",
+            "documents.locale",
+            "editions.id as edition_id",
+            "links.position",
+            "editions.state",
+            "lookahead.links",
+          )
+      end
+    end
+  end
+end

app/queries/recursive_link_expansion/link_expansion_rules.rb

@@ -0,0 +1,22 @@
+module Queries
+  module RecursiveLinkExpansion
+    class LinkExpansionRules
+      RULES_BY_SCHEMA_NAME = {
+        ministers_index: [
+          { "type": "ordered_cabinet_ministers", "reverse": false, "links": [{ "type": "person", "reverse": true, "links": [{ "type": "role", "reverse": false, "links": [] }] }] },
+          { "type": "ordered_also_attends_cabinet", "reverse": false, "links": [{ "type": "person", "reverse": true, "links": [{ "type": "role", "reverse": false, "links": [] }] }] },
+          { "type": "ordered_ministerial_departments", "reverse": false, "links": [{ "type": "ordered_ministers", "reverse": false, "links": [{ "type": "person", "reverse": true, "links": [{ "type": "role", "reverse": false, "links": [{ "type": "ordered_roles", "reverse": true, "links": [] }] }] }] }] },
+          { "type": "ordered_assistant_whips", "reverse": false, "links": [{ "type": "person", "reverse": true, "links": [{ "type": "role", "reverse": false, "links": [] }] }] },
+          { "type": "ordered_baronesses_and_lords_in_waiting_whips", "reverse": false, "links": [{ "type": "person", "reverse": true, "links": [{ "type": "role", "reverse": false, "links": [] }] }] },
+          { "type": "ordered_house_lords_whips", "reverse": false, "links": [{ "type": "person", "reverse": true, "links": [{ "type": "role", "reverse": false, "links": [] }] }] },
+          { "type": "ordered_house_of_commons_whips", "reverse": false, "links": [{ "type": "person", "reverse": true, "links": [{ "type": "role", "reverse": false, "links": [] }] }] },
+          { "type": "ordered_junior_lords_of_the_treasury_whips", "reverse": false, "links": [{ "type": "person", "reverse": true, "links": [{ "type": "role", "reverse": false, "links": [] }] }] },
+        ],
+      }.with_indifferent_access.freeze
+
+      def self.for(schema_name)
+        RULES_BY_SCHEMA_NAME.fetch(schema_name)
+      end
+    end
+  end
+end

app/queries/recursive_link_expansion/lookahead.rb

@@ -0,0 +1,41 @@
+module Queries
+  module RecursiveLinkExpansion
+    ##
+    # Expects to be called with a CTE called "linked_editions" already defined.
+    # Defines a "lookahead" from a content id or an edition id and a link type.
+    #
+    # This allows the various types of link query (forward / reverse link set / edition links) to
+    # follow the link type from the edition / content id to the next edition / content_id.
+    #
+    # links is required in the constructor because we need to provide the maximum number of links that
+    # this CTE could return, as otherwise the postgresql planner will assume jsonb_to_recordset() will return 100 rows
+    # which pushes it towards inefficient plans.
+    class Lookahead
+      def initialize(links)
+        @links = links.map(&:deep_symbolize_keys)
+      end
+
+      def call
+        Arel.sql(<<~SQL,
+          SELECT content_id, edition_id, path, lookahead.type, lookahead.reverse, lookahead.links
+          FROM linked_editions
+          CROSS JOIN LATERAL (
+            SELECT * from jsonb_to_recordset(linked_editions.links) AS lookahead(type varchar, reverse boolean, links jsonb)
+            LIMIT #{max_links_count(@links)}
+          ) AS lookahead
+        SQL
+                )
+      end
+
+    private
+
+      def max_links_count(links)
+        case links
+        in { links: Array => ls } then max_links_count(ls)
+        in Hash | [] then 0
+        in Array then [links.length, links.map { max_links_count(_1) }.max].max
+        end
+      end
+    end
+  end
+end

app/queries/recursive_link_expansion/recursive_link_expansion.rb

@@ -0,0 +1,35 @@
+module Queries
+  module RecursiveLinkExpansion
+    class RecursiveLinkExpansion
+      def initialize(edition)
+        @edition = edition
+        @links = ::Queries::RecursiveLinkExpansion::LinkExpansionRules.for(edition.schema_name)
+      end
+
+      def call
+        base_case = BaseEdition.new(@edition, @links).call
+
+        recursive_case = Edition.with(
+          lookahead: Lookahead.new(@links).call,
+          forward_link_set_links: ForwardLinkSetLinks.new.call,
+          forward_edition_links: ForwardEditionLinks.new.call,
+          reverse_link_set_links: ReverseLinkSetLinks.new.call,
+          reverse_edition_links: ReverseEditionLinks.new.call,
+          all_links: [
+            Arel.sql("SELECT * from forward_link_set_links"),
+            Arel.sql("SELECT * from forward_edition_links"),
+            Arel.sql("SELECT * from reverse_link_set_links"),
+            Arel.sql("SELECT * from reverse_edition_links"),
+          ],
+        ).from("all_links").select("all_links.*")
+
+        Edition.with_recursive(
+          linked_editions: [
+            base_case,
+            Arel.sql(recursive_case.to_sql), # Workaround to ensure the recursive case is wrapped in parens
+          ],
+        ).from("linked_editions").select("linked_editions.*")
+      end
+    end
+  end
+end

app/queries/recursive_link_expansion/reverse_edition_links.rb

@@ -0,0 +1,24 @@
+module Queries
+  module RecursiveLinkExpansion
+    class ReverseEditionLinks
+      def call
+        # TODO: - parameterize locale / state
+        Edition
+          .from("lookahead")
+          .joins("INNER JOIN links ON (links.target_content_id = lookahead.content_id AND links.link_type = lookahead.type AND lookahead.reverse = true)")
+          .joins("INNER JOIN editions ON (editions.id = links.edition_id AND editions.state='published')")
+          .joins("INNER JOIN documents ON (documents.id = editions.document_id AND documents.locale='en')")
+          .select(
+            "'1 forward edition link' as type",
+            "path || lookahead.content_id::text || lookahead.type AS path",
+            "documents.content_id",
+            "documents.locale",
+            "editions.id as edition_id",
+            "links.position",
+            "editions.state",
+            "lookahead.links",
+          )
+      end
+    end
+  end
+end

app/queries/recursive_link_expansion/reverse_link_set_links.rb

@@ -0,0 +1,24 @@
+module Queries
+  module RecursiveLinkExpansion
+    class ReverseLinkSetLinks
+      def call
+        # TODO: - parameterize locale / state
+        Edition
+          .from("lookahead")
+          .joins("INNER JOIN links ON links.target_content_id = lookahead.content_id and links.link_type = lookahead.type and lookahead.reverse = true")
+          .joins("INNER JOIN documents ON documents.content_id = links.link_set_content_id and documents.locale IN ('en')")
+          .joins("INNER JOIN editions ON editions.document_id = documents.id AND editions.state IN ('published')")
+          .select(
+            "'4 reverse link set link' as type",
+            "path || lookahead.content_id::text || lookahead.type as path",
+            "documents.content_id",
+            "documents.locale",
+            "editions.id as edition_id",
+            "links.position as position",
+            "editions.state as state",
+            "lookahead.links",
+          )
+      end
+    end
+  end
+end