Add topological sorting for dumped views using TSort

Co-authored-by: Mykhaylo Sorochan <mykhaylo.sorochan@toptal.com>
Co-authored-by: Edward Loveall <edward@edwardloveall.com>

lib/scenic/schema_dumper.rb

@@ -3,6 +3,34 @@
 module Scenic
   # @api private
   module SchemaDumper
+    # A hash to do topological sort
+    class TSortableHash < Hash
+      include TSort
+
+      alias_method :tsort_each_node, :each_key
+      def tsort_each_child(node, &)
+        fetch(node).each(&)
+      end
+    end
+
+    # Query for the dependencies between views
+    DEPENDENT_SQL = <<~SQL.freeze
+      SELECT distinct dependent_ns.nspname AS dependent_schema
+      , dependent_view.relname AS dependent_view
+      , source_ns.nspname AS source_schema
+      , source_table.relname AS source_table
+      FROM pg_depend
+      JOIN pg_rewrite ON pg_depend.objid = pg_rewrite.oid
+      JOIN pg_class as dependent_view ON pg_rewrite.ev_class = dependent_view.oid
+      JOIN pg_class as source_table ON pg_depend.refobjid = source_table.oid
+      JOIN pg_namespace dependent_ns ON dependent_ns.oid = dependent_view.relnamespace
+      JOIN pg_namespace source_ns ON source_ns.oid = source_table.relnamespace
+      WHERE dependent_ns.nspname = ANY (current_schemas(false)) AND source_ns.nspname = ANY (current_schemas(false))
+      AND source_table.relname != dependent_view.relname
+      AND source_table.relkind IN ('m', 'v') AND dependent_view.relkind IN ('m', 'v')
+      ORDER BY dependent_view.relname;
+    SQL
+
     def tables(stream)
       super
       views(stream)
@@ -22,11 +50,47 @@ def views(stream)
     private
 
     def dumpable_views_in_database
-      @dumpable_views_in_database ||= Scenic.database.views.reject do |view|
-        ignored?(view.name)
+      @ordered_dumpable_views_in_database ||= begin
+        existing_views = Scenic.database.views.reject do |view|
+          ignored?(view.name)
+        end
+
+        tsorted_views(existing_views.map(&:name)).map do |view_name|
+          existing_views.find do |ev|
+            ev.name == view_name || ev.name == view_name.split(".").last
+          end
+        end.compact
       end
     end
 
+    # When dumping the views, their order must be topologically
+    # sorted to take into account dependencies
+    def tsorted_views(views_names)
+      views_hash = TSortableHash.new
+
+      ::Scenic.database.execute(DEPENDENT_SQL).each do |relation|
+        source_v = [
+          relation["source_schema"],
+          relation["source_table"]
+        ].compact.join(".")
+        dependent = [
+          relation["dependent_schema"],
+          relation["dependent_view"]
+        ].compact.join(".")
+        views_hash[dependent] ||= []
+        views_hash[source_v] ||= []
+        views_hash[dependent] << source_v
+        views_names.delete(relation["source_table"])
+        views_names.delete(relation["dependent_view"])
+      end
+
+      # after dependencies, there might be some views left
+      # that don't have any dependencies
+      views_names.sort.each { |v| views_hash[v] ||= [] }
+
+      views_hash.tsort
+    end
+
     unless ActiveRecord::SchemaDumper.private_instance_methods(false).include?(:ignored?)
       # This method will be present in Rails 4.2.0 and can be removed then.
       def ignored?(table_name)