Can't save flat rate calculator because of Psych::DisallowedClass

[winstonferguson]

Encountered when trying to create a flat rate calculator in the Admin namespace.

As I have another project using the spree_backend gem without this issue, I'm reporting it here.

Context

This is a fresh spree_rails_frontend install (4.8.0). On a local machine using Rbenv.

Expected Behavior

Calculator is saved.

Actual Behavior

Psych::DisallowedClass in Spree::Admin::ShippingMethodsController#update
Tried to dump unspecified class: BigDecimal

/.rbenv/versions/3.3.0/lib/ruby/3.3.0/psych/visitors/yaml_tree.rb:571:in accept' /.rbenv/versions/3.3.0/lib/ruby/3.3.0/psych/visitors/yaml_tree.rb:331:in block in visit_Hash'
/.rbenv/versions/3.3.0/lib/ruby/3.3.0/psych/visitors/yaml_tree.rb:329:in each' /.rbenv/versions/3.3.0/lib/ruby/3.3.0/psych/visitors/yaml_tree.rb:329:in visit_Hash'
/.rbenv/versions/3.3.0/lib/ruby/3.3.0/psych/visitors/yaml_tree.rb:135:in accept' /.rbenv/versions/3.3.0/lib/ruby/3.3.0/psych/visitors/yaml_tree.rb:574:in accept'
/.rbenv/versions/3.3.0/lib/ruby/3.3.0/psych/visitors/yaml_tree.rb:117:in push' /.rbenv/versions/3.3.0/lib/ruby/3.3.0/psych.rb:585:in safe_dump'
activerecord (7.1.3.4) lib/active_record/coders/yaml_column.rb:19:in `dump'

Possible Fix

Ensure class is permitted.

Add:

config.active_record.yaml_column_permitted_classes << BigDecimal

Reference:

rails/rails#45826
https://stackoverflow.com/questions/74312283/tried-to-load-unspecified-class-activesupporttimewithzone-psychdisallowed

[FrederikKragh]

I am encountering the same error when creating a flat rate calculator for my Shipping Method. In addition I am encountering it when trying to add a flat rate Action to my promotion

Psych::DisallowedClass in Spree::Admin::PromotionsController#update

[bjornlinder]

Also ran into Disallowed class error on attempting to create a flat shipping rate.

Spree::ShippingMethod Create (1.5ms)  INSERT INTO "spree_shipping_methods" ("name", "display_on", "deleted_at", "created_at", "updated_at", "tracking_url", "admin_name", "tax_category_id", "code", "public_metadata", "private_metadata") VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11) RETURNING "id"  [["name", "Standard"], ["display_on", "both"], ["deleted_at", nil], ["created_at", "2025-06-25 19:08:58.997024"], ["updated_at", "2025-06-25 19:08:58.997024"], ["tracking_url", ""], ["admin_name", ""], ["tax_category_id", 1], ["code", ""], ["public_metadata", nil], ["private_metadata", nil]]
[6f03eec7-5828-4d89-b665-41a91f1458b2]   TRANSACTION (0.3ms)  ROLLBACK

[6f03eec7-5828-4d89-b665-41a91f1458b2] Psych::DisallowedClass (Tried to dump unspecified class: Symbol(:minimum_item_total)):
[6f03eec7-5828-4d89-b665-41a91f1458b2]   
[6f03eec7-5828-4d89-b665-41a91f1458b2] /usr/local/rvm/rubies/ruby-3.3.0/lib/ruby/3.3.0/psych/visitors/yaml_tree.rb:579:in `visit_Symbol'

[bjornlinder]

Found the solution in /core/lib/generators/spree/dummy/dummy_generator.rb, manually added this line to application.rb:
config.active_record.yaml_column_permitted_classes = [Symbol, BigDecimal, ActiveSupport::HashWithIndifferentAccess]. I was only experiencing the error in production, shipping methods worked fine in development. The error likely arose from that config array being loaded incorrectly in production, idk how but prod console showed a double array where dev showed a single array:

ActiveRecord.yaml_column_permitted_classes
 => [[Symbol, BigDecimal, ActiveSupport::HashWithIndifferentAccess]]