docs: add testing philosophy to coding guideline

and update groups architecture docs #371

docs/groups-architecture.md

@@ -944,6 +944,24 @@ Each functional unit can be implemented as a **separate issue**:
 
 ## Testing Strategy
 
+### Testing Philosophy
+
+**Focus on Business Logic, Not Framework Functionality**
+
+We test our business logic and domain-specific behavior, not core framework features. Framework features (Ash validations, Ecto relationships, etc.) are already tested by their respective libraries.
+
+**What We Test:**
+- Business rules and validations specific to our domain
+- Custom business logic (slug generation, calculations, etc.)
+- Integration between our resources
+- Database-level constraints (unique constraints, foreign keys, CASCADE)
+- Query performance (N+1 prevention)
+
+**What We Don't Test:**
+- Framework core functionality (Ash validations work, Ecto relationships work, etc.)
+- Standard CRUD operations without custom logic
+- Framework-provided features that are already tested upstream
+
 ### Unit Tests
 
 #### Group Resource Tests
@@ -952,31 +970,23 @@ Each functional unit can be implemented as a **separate issue**:
 
 **Test Cases:**
 - Create group with valid attributes
-- Return error when name is missing
-- Return error when name exceeds 100 characters
-- Return error when name is not unique
-- Name uniqueness is case-insensitive
-- Allow description to be nil
-- Trim whitespace from name
-- Description max length is 500 characters
-- Slug is automatically generated from name on create
-- Slug is immutable (doesn't change on update)
-- Slug is unique (cannot have duplicates)
-- Slug handles German umlauts correctly (ä → a, ß → ss)
-- Slug converts to lowercase
-- Slug removes special characters
-- Slug replaces spaces with hyphens
-- Slug truncates to max 100 characters
-- Slug cannot be empty (validation fails if slug would be empty)
-- Read group by slug (via identity lookup)
+- Return error when name is missing (required validation)
+- Return error when name exceeds 100 characters (length validation)
+- Return error when name is not unique (case-insensitive) - application level validation
+- Allow description to be nil (optional field)
+- Description max length is 500 characters (length validation)
+- Slug is automatically generated from name on create (custom business logic)
+- Slug is immutable (doesn't change when name is updated) - business rule
+- Slug is unique (prevents duplicate slugs from different names) - business rule
+- Slug cannot be empty (rejects name with only special characters) - business rule
 - Update group name and description
-- Prevent duplicate name on update
-- Prevent duplicate slug on create (same name → same slug → error)
-- Delete group and all member associations
-- Do not delete members themselves
-- Member count calculation returns 0 for empty group
-- Member count calculation returns correct count when members added
-- Member count updates correctly when members removed
+- Prevent duplicate name on update (case-insensitive) - business rule
+- Delete group and all member associations (cascade behavior)
+- Do not delete members themselves (cascade boundary)
+- Member count calculation returns 0 for empty group (custom calculation)
+- Member count calculation returns correct count when members added/removed (custom calculation)
+
+**Note:** Detailed slug generation tests (Umlauts, truncation, etc.) are covered by the `GenerateSlug` change tests in `custom_field_slug_test.exs`, which is reused for groups. We don't duplicate these framework-level tests.
 
 #### MemberGroup Resource Tests
 
@@ -984,9 +994,23 @@ Each functional unit can be implemented as a **separate issue**:
 
 **Test Cases:**
 - Create association between member and group
-- Prevent duplicate associations
-- Cascade delete when member deleted
-- Cascade delete when group deleted
+- Prevent duplicate associations (unique constraint)
+- Cascade delete when member deleted (database constraint)
+- Cascade delete when group deleted (database constraint)
+
+#### Database Constraint Tests
+
+**File:** `test/membership/group_database_constraints_test.exs`
+
+**Test Cases:**
+- Database enforces unique name constraint (case-insensitive via LOWER) - DB level
+- Database enforces unique slug constraint (case-sensitive) - DB level
+- Cannot create MemberGroup with non-existent member_id (foreign key constraint)
+- Cannot create MemberGroup with non-existent group_id (foreign key constraint)
+- Deleting member cascades to member_groups (verified at DB level)
+- Deleting group cascades to member_groups (verified at DB level)
+
+**Note:** These tests verify that constraints are enforced at the database level, not just application level.
 
 ### Integration Tests
 
@@ -995,8 +1019,20 @@ Each functional unit can be implemented as a **separate issue**:
 **File:** `test/membership/group_integration_test.exs`
 
 **Test Cases:**
-- Member can belong to multiple groups
-- Group can contain multiple members
+- Member can belong to multiple groups (many-to-many relationship)
+- Group can contain multiple members (many-to-many relationship)
+- Preloading groups with members avoids N+1 queries (performance test with query count verification)
+
+**File:** `test/membership/member_groups_relationship_test.exs`
+
+**Test Cases:**
+- Member has many_to_many groups relationship (load with preloading)
+- Load multiple members with groups preloaded (N+1 prevention)
+- Add member to group via Ash API
+- Remove member from group via Ash API
+- Add member to multiple groups in single operation
+- Adding member to same group twice fails (duplicate prevention)
+- Removing member from group they're not in (idempotent, no error)
 
 ### UI Tests