Support explicitly specifying :use_placeholder_loader association option

In general, this should only be explicitly set to false to disable the
use of the loader (useful for easier testing), it should not be
explicitly set to true.

CHANGELOG

@@ -1,5 +1,7 @@
 === master
 
+* Support explicitly specifying :use_placeholder_loader association option (jeremyevans)
+
 * Make SQL::BooleanExpression.from_value_pairs handle Dataset::PlaceholderLiteralizer::Argument with existing transformer correctly (jeremyevans)
 
 * Make connection_validator extension not swallow exceptions raised by Database#valid_connection? (jeremyevans)

lib/sequel/model/associations.rb

@@ -1769,6 +1769,9 @@ def all_association_reflections
         #            Set to nil to not define a setter method for the association.
         # :subqueries_per_union :: The number of subqueries to use in each UNION query, for eager
         #                          loading limited associations using the default :union strategy.
+        # :use_placeholder_loader :: Whether to use a placeholder loader when eager loading the
+        #                            association.  Can be set to false to disable the use of a placeholder
+        #                            loader if one would be used by default.
         # :validate :: Set to false to not validate when implicitly saving any associated object.
         # === :many_to_one
         # :key :: foreign key in current model's table that references
@@ -1891,7 +1894,7 @@ def associate(type, name, opts = OPTS, &block)
             raise(Error, "cannot clone an association to an association of different type (association #{name} with type #{type} cloning #{opts[:clone]} with type #{cloned_assoc[:type]})")
           end
 
-          opts[:use_placeholder_loader] = !opts[:instance_specific] && !opts[:eager_graph]
+          opts[:use_placeholder_loader] = !opts[:instance_specific] && !opts[:eager_graph] unless opts.include?(:use_placeholder_loader)
           opts[:eager_block] = opts[:block] unless opts.include?(:eager_block)
           opts[:graph_join_type] ||= :left_outer
           opts[:order_eager_graph] = true unless opts.include?(:order_eager_graph)