Spree Dependencies (Dependency Injection)

Commands below use the Spree CLI form (spree …, Docker). On a classic Rails app without the CLI (typical pre-5.4), use the native mapping in the spree-project skill — bin/rails / bundle exec rake from the app root, paths without the backend/ prefix.

Spree.dependencies is the canonical way to replace a core Spree service with your own implementation — no fork, no monkey-patch, no decorator. You inherit from the Spree default, override the methods you need, and register your class as the dependency. Spree’s own code calls your service everywhere it used to call the default.

The core has 70 injection points; the API has 302 more for serializers, finders, and per-endpoint services. The full set is documented at node_modules/@spree/docs/dist/developer/customization/dependencies.md.

When to reach for this vs other patterns

If Spree gives you a swappable service, use it. Decorating Spree::Cart::AddItem would couple you to the parent’s step names and break on minor upgrades; subclassing + injection is the supported extension point.

The basic pattern

Step 1: subclass the Spree default

# app/services/my_app/cart/add_item.rb
class MyApp::Cart::AddItem < Spree::Cart::AddItem
  def call(order:, variant:, quantity: nil, metadata: {}, public_metadata: {}, private_metadata: {}, options: {})
    ApplicationRecord.transaction do
      run :add_to_line_item
      run :handle_stock_reservations            # keep the parent's steps you need
      run :update_in_external_system            # your custom step
      run Spree.cart_recalculate_service        # call other dependencies dynamically
    end
  end

  private

  def update_in_external_system(order:, line_item:, **rest)
    # Your custom logic
    success(order: order, line_item: line_item, **rest)
  end
end

Inherit from the Spree default. Override call if you need to change the step chain; override individual private steps (add_to_line_item, handle_stock_reservations) if you only need to tweak one piece of behavior.

Every run step must return success(...) or failure(...) — otherwise Spree::ServiceModule::WrongDataPassed is raised after the chain. The value you pass to success is double-splatted (**) into the next step, so it must be a hash whenever another step follows. That’s also why the custom step goes before run Spree.cart_recalculate_service here: Spree::Cart::Recalculate ends with success(line_item) (a bare LineItem, not a hash), so no run step can come after it.

Step 2: register the override

In config/initializers/spree.rb. Two syntaxes — both work, the direct form is concise:

# Direct (recommended)
Spree.cart_add_item_service = MyApp::Cart::AddItem

# Block (when you're setting several at once)
Spree.dependencies do |deps|
  deps.cart_add_item_service       = MyApp::Cart::AddItem
  deps.cart_recalculate_service    = MyApp::Cart::Recalculate
  deps.checkout_advance_service    = MyApp::Checkout::Advance
end

Step 3: Spree picks up your service everywhere

You don’t have to find and patch callers. Spree’s own code calls Spree.cart_add_item_service.call(...) rather than Spree::Cart::AddItem.call(...) — your replacement runs in admin requests, API requests, the dashboard, the storefront, background jobs, everything.

Using dependencies from your own code

When your code needs to call a Spree service that might be overridden by someone else’s customization, go through the dependency accessor — never hardcode the default class:

# ✅ Resolves to the current dependency (default OR override)
Spree.cart_add_item_service.call(order: order, variant: variant, quantity: 1)

Spree.api.cart_serializer.new(order).serializable_hash

# ❌ Bypasses any override another extension or app initializer set
Spree::Cart::AddItem.call(order: order, variant: variant, quantity: 1)

This matters for extensions and shared code — using the accessor means your code composes cleanly with whatever overrides the host app has registered.

Global vs API-level overrides

The two API surfaces are Store API v3 and Admin API v3. They share core services but have separate serializer injection points, so you can customize one surface without touching the other:

• Serializers are per-surface: Spree.api.<resource>_serializer for the Store API (e.g. Spree.api.product_serializer, Spree.api.cart_serializer) and Spree.api.admin_<resource>_serializer for the Admin API (e.g. Spree.api.admin_product_serializer). Admin serializers extend their Store counterparts, so public-field changes propagate automatically.
• Services have a single core-level injection point — Spree.<name> (e.g. Spree.cart_add_item_service). All v3 endpoints, Store and Admin alike, call the core dependency directly; there is no per-surface service layer.

# Service swap — affects every caller: Store API, Admin API, jobs, custom code
Spree.cart_add_item_service = MyApp::CartAddItem

# Store API serializer only — Admin API keeps its own
Spree.api.product_serializer = 'MyApp::ProductSerializer'

# Admin API serializer only
Spree.api.admin_product_serializer = 'MyApp::Admin::ProductSerializer'

Warning: Spree.api still defines legacy storefront_* and platform_* injection points (e.g. Spree.api.storefront_cart_add_item_service), and they still appear in spree rake spree:dependencies:list output under [API]. These belonged to the removed API v2 controllers and have no consumers — setting them is a silent no-op and they will be removed in Spree 6. Always use the global Spree.<name> service accessors instead.

Per-controller overrides

If you only want to swap a serializer for one specific controller (rather than globally or per-surface), use a controller decorator overriding serializer_class:

# app/controllers/spree/api/v3/store/carts_controller_decorator.rb
module Spree::Api::V3::Store
  module CartsControllerDecorator
    def serializer_class
      MyApp::PremiumCartSerializer
    end
  end
end

Spree::Api::V3::Store::CartsController.prepend Spree::Api::V3::Store::CartsControllerDecorator

Generate the file with bin/rails g spree:controller_decorator Spree::Api::V3::Store::CartsController.

The v3 ResourceController hooks you can override this way are model_class, serializer_class, scope, find_resource, permitted_params, and collection_includes. There are no per-controller <name>_service hooks in API v3 — controllers call the registered dependencies directly, so to swap a service use the global (Spree.<name>) injection points described above. (Per-surface Spree.api.* points exist only for serializers.)

For the decorator syntax + generator, see the spree-decorators skill.

Inspecting + debugging dependencies

Spree ships three rake tasks for working with the dependency graph.

List everything

spree rake spree:dependencies:list

Output looks like:

[CORE]
ability_class                    Spree::Ability
cart_add_item_service            Spree::Cart::AddItem
cart_create_service              Spree::Cart::Create
cart_recalculate_service         Spree::Cart::Recalculate [OVERRIDDEN]
...

[API]
cart_serializer                  Spree::Api::V3::CartSerializer
admin_product_serializer         Spree::Api::V3::Admin::ProductSerializer
product_serializer               MyApp::ProductSerializer [OVERRIDDEN]
...

[OVERRIDDEN] flags every dependency that’s been swapped from the default — invaluable for figuring out what an extension changed.

Filter to find a specific service:

spree rake spree:dependencies:list | grep cart
spree rake spree:dependencies:list | grep -i serializer

Show only overrides

spree rake spree:dependencies:overrides

Lists only the dependencies that differ from the default, with the source location (file + line) of the override:

[Core OVERRIDES]
cart_recalculate_service         Spree::Cart::Recalculate -> MyApp::Cart::Recalculate (config/initializers/spree.rb:15)

[API OVERRIDES]
storefront_cart_add_item_service Spree::Cart::AddItem     -> MyApp::CartAddItem        (config/initializers/spree.rb:20)

Use this when you walk into an inherited project — it answers “what has this app customized?” in one command.

Validate that all dependencies resolve

spree rake spree:dependencies:validate

Loads every registered dependency and confirms it points to a real class. Catches typos and missing constants before runtime:

....F...............
1 invalid dependencies:
  [Core] cart_add_item_service: uninitialized constant MyApp::Cart::AddIem

Wire this into CI on any project that overrides dependencies. Typos here are silent at boot and only surface when the affected code path runs in production.

Programmatic introspection

The rake tasks are thin wrappers around a public Ruby API. Use it in console sessions, custom rake tasks, or extension health checks:

# All dependencies with their current + default values
Spree::Dependencies.current_values
# => [{name: :cart_add_item_service, current: MyApp::CartAddItem, default: 'Spree::Cart::AddItem', overridden: true}, ...]

# Is a specific dependency overridden?
Spree::Dependencies.overridden?(:cart_add_item_service)
# => true

# Where was the override set?
Spree::Dependencies.override_info(:cart_add_item_service)
# => {value: MyApp::CartAddItem, source: "config/initializers/spree.rb:15", set_at: 2024-01-15 10:30:00}

# Validate — checks every injection point and raises Spree::DependencyError listing all bad references
Spree::Dependencies.validate!

Spree::Api::Dependencies exposes the same surface for the API-level injection points.

The catalog — what’s actually swappable

The injection points are grouped by domain. The list is too long to enumerate in full; this is the categorical map. Run spree rake spree:dependencies:list to see the full set for the installed version.

Core (70 injection points)

API (302 injection points)

The Spree.api.<name> accessor reaches into API-level dependencies; the bare Spree.<name> accessor reaches into core dependencies.

Backwards compatibility — old syntax

Older Spree projects used a string-based syntax that’s still supported:

# Legacy (still works)
Spree::Dependencies.cart_add_item_service = 'MyApp::Cart::AddItem'
result = Spree::Dependencies.cart_add_item_service.constantize

# New (recommended — concise, fails at assignment if the class doesn't exist)
Spree.cart_add_item_service = MyApp::Cart::AddItem
result = Spree.cart_add_item_service

Both can coexist in one initializer. New code should use the direct syntax — it catches misspelled class names immediately rather than at first invocation.

Common pitfalls

Calling the wrong accessor in your own code

Spree.cart_add_item_service resolves to whatever the current dependency is (default OR an override). Spree::Cart::AddItem is always the literal default class. Always use the accessor in code that might run alongside an override — extensions, shared services, decorators. Hardcoding the default class breaks composition.

Forgetting to inherit from the default

A common mistake on first try:

# ❌ Doesn't inherit — your class is missing Spree's service module wiring
class MyApp::Cart::AddItem
  def call(order:, variant:, **)
    # ...
  end
end

# ✅ Inherits Spree::ServiceModule::Base via the parent
class MyApp::Cart::AddItem < Spree::Cart::AddItem
  def call(order:, variant:, **)
    super  # or compose your own steps
  end
end

Spree services prepend Spree::ServiceModule::Base for the run step orchestration. Your replacement must inherit from the Spree default (or prepend that module itself — include will not wire the class-level .call) for the same step-chain behavior to work.

Dropping steps from call

When you override call, every run step in the parent that you don’t repeat is dropped:

# Parent:
def call(order:, variant:, **)
  ApplicationRecord.transaction do
    run :add_to_line_item
    run :handle_stock_reservations       # ← parent has this step
    run Spree.cart_recalculate_service
  end
end

# Bad override — drops handle_stock_reservations silently:
def call(order:, variant:, **)
  ApplicationRecord.transaction do
    run :add_to_line_item
    run Spree.cart_recalculate_service
    run :my_custom_step
  end
end

Stock reservations stop working. Read the parent’s call and preserve every step you don’t have a reason to drop.

Assuming you need a second, API-level assignment

You don’t. A global override applies everywhere, including the v3 Store and Admin APIs:

Spree.cart_add_item_service = MyApp::Cart::AddItem  # ← this is all you need

The v3 controllers call Spree.cart_add_item_service (the core injection point) directly and resolve it lazily at request time, so an override in config/initializers/spree.rb takes effect for API requests too.

The Spree.api.storefront_* service points are leftovers from the removed API v2 (marked “Legacy API v2 dependencies — will be removed in Spree 6” in spree_api/lib/spree/api/dependencies.rb). Nothing consumes them anymore — assigning Spree.api.storefront_cart_add_item_service is a no-op. Don’t rely on their “cascade” either: their proc defaults are snapshotted once, when Spree::Api::Dependencies is instantiated in an engine initializer that runs before your app’s initializers — so core overrides set in config/initializers/spree.rb never propagate into them. They’ll still appear in spree:dependencies:list output showing the stale boot-time value; ignore them.

Initializer load order

Dependency overrides go in config/initializers/spree.rb. Multiple extensions setting the same dependency follow alphabetical gem load order — the last assignment wins. If two gems both try to override cart_add_item_service, only the alphabetically-later one’s override survives. Use spree:dependencies:overrides to confirm what actually ended up registered.

Where to read further

• Canonical docs: node_modules/@spree/docs/dist/developer/customization/dependencies.md
• Core injection point list: Spree::Core::Dependencies::INJECTION_POINTS_WITH_DEFAULTS in spree_core/lib/spree/core/dependencies.rb
• API injection point list: Spree::Api::ApiDependencies::INJECTION_POINTS_WITH_DEFAULTS in spree_api/lib/spree/api/dependencies.rb (Spree::Api::Dependencies is an instance of this class)
• Spree::ServiceModule::Base — the base class behind the run :step_name orchestration
• For deciding whether to swap a service vs use events vs decorate: the spree-customization skill
• For installing third-party Spree gems that ship dependency overrides: the spree-extensions skill