feat: replace SqliteProjectionRepo with RepoFamily GAT

[0xgleb]

Closes RAI-465.

What

Introduces a ViewBackend trait and SqliteViewBackend struct, replacing the previous SqliteProjectionRepo newtype. Projection is now generic over Backend: ViewBackend instead of a raw Repo type parameter. SqliteProjectionRepo is removed from the public API, and SqliteViewBackend is exported in its place alongside the new ViewBackend trait.

Why

The previous design used SqliteProjectionRepo as a newtype solely to hide the pub(crate) Lifecycle type from public bounds on Projection. This was a leaky workaround that required #[allow(private_bounds)] suppressions and left a TODO about proper higher-kinded type (HKT) support. The new ViewBackend trait uses a GAT (type Repo<View, Agg>) to emulate HKT, allowing Projection to resolve its repository type internally without ever surfacing Lifecycle in public where clauses. This removes the need for the #[allow(private_bounds)] suppressions and makes the backend genuinely pluggable.

How

A new view_backend module defines ViewBackend as a trait with a single GAT Repo<View, Agg>, and SqliteViewBackend as the default implementation mapping every (View, Agg) pair to SqliteViewRepository. Projection replaces its Repo type parameter with Backend: ViewBackend and stores Arc<Backend::Repo<Lifecycle<Entity>, Lifecycle<Entity>>> directly. The 'static bound is added to EventSourced to satisfy the GAT constraints. Test helpers (InMemoryRepo, ConflictingRepo) are made generic over (View, Agg) and wrapped in corresponding InMemoryViewBackend and ConflictingViewBackend impls of ViewBackend.

Summary by CodeRabbit

• Breaking Changes

  • The Projection API now uses a pluggable backend abstraction instead of a fixed repository type, requiring updates to generic parameters in client code.
  • A previously exported repository type is no longer publicly available; applications must use the new backend abstraction.

• New Features

  • Introduced a new abstraction layer enabling custom view storage backends for greater flexibility.

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📥 Commits

Reviewing files that changed from the base of the PR and between f31b57c and 0845e34.

⛔ Files ignored due to path filters (2)

• examples/complex/Cargo.lock is excluded by !**/*.lock
• examples/simple/Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (24)

• .github/workflows/ci.yaml
• Cargo.toml
• README.md
• crates/event-sorcery/src/lib.rs
• crates/event-sorcery/src/projection.rs
• crates/event-sorcery/src/view_backend.rs
• examples/README.md
• examples/complex/Cargo.toml
• examples/complex/README.md
• examples/complex/migrations/20260513104032_init.sql
• examples/complex/migrations/20260513104033_inventory_view.sql
• examples/complex/src/audit_log.rs
• examples/complex/src/inventory.rs
• examples/complex/src/main.rs
• examples/complex/src/order.rs
• examples/complex/src/stock_alert.rs
• examples/simple/Cargo.toml
• examples/simple/README.md
• examples/simple/migrations/20260513104023_init.sql
• examples/simple/migrations/20260513104024_support_ticket_view.sql
• examples/simple/src/main.rs
• examples/simple/src/support_ticket.rs
• flake.nix
• scripts/check-examples.nu

Walkthrough

This PR refactors the materialized-view storage layer in event-sorcery from a concrete SqliteProjectionRepo wrapper to a pluggable ViewBackend trait with Generic Associated Types (GATs). Projection is now generic over Backend: ViewBackend instead of a concrete repository type. Test infrastructure is generalized to support multiple backends, and public API exports are updated accordingly.

Changes

ViewBackend Abstraction and Projection Refactor

Layer / File(s)	Summary
View Backend Abstraction crates/event-sorcery/src/view_backend.rs	Introduces ViewBackend trait with a GAT type Repo<View, Agg> for mapping view/aggregate pairs to concrete repositories. Provides SqliteViewBackend as the default implementation.
Projection Struct and Core Methods crates/event-sorcery/src/projection.rs (doc, struct, constructors)	Changes Projection from Projection<Entity, Repo = SqliteProjectionRepo<Entity>> to Projection<Entity, Backend: ViewBackend = SqliteViewBackend>. Updates sqlite() and new() to work with backend-associated repo types.
Projection Trait Implementations crates/event-sorcery/src/projection.rs (Clone, Dependent, Reactor)	Clone, Dependent, and Reactor trait bounds are expressed via Backend: ViewBackend instead of direct repository requirements.
Generalized Test Repositories crates/event-sorcery/src/projection.rs (test modules)	InMemoryRepo<View, Agg> and ConflictingRepo<View, Agg> are generalized from Lifecycle<TestEntity>-specific implementations. Test backend adapters InMemoryViewBackend and ConflictingViewBackend implement the ViewBackend trait.
Test Case Wiring crates/event-sorcery/src/projection.rs (test functions)	Existing behavioral tests are updated to construct Projection instances via backend-typed repos and the Projection::new() constructor.
Public API Exports crates/event-sorcery/src/lib.rs	Declares the new view_backend module and updates re-exports: removes SqliteProjectionRepo, adds ViewBackend and SqliteViewBackend. Reformats EventSourced trait declaration (whitespace only).

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically describes the main change: replacing SqliteProjectionRepo with a RepoFamily GAT-based abstraction (ViewBackend trait), which aligns with the PR's core objective of refactoring the projection repository abstraction.
Docstring Coverage	✅ Passed	Docstring coverage is 86.36% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[0xgleb]

• adr: jobs replace services #8
• docs: add examples #7
• feat: replace SqliteProjectionRepo with RepoFamily GAT #6 👈 (View in Graphite)
• feat: GitHub Actions CI #5
• feat: AGENTS.md, README, and docs (cqrs, sqlx, ttdd) #4
• feat: copy sqlite-es and event-sorcery crates #3
• feat: nix flake #2
• event-sorcery and sqlite-es #1
• master

This stack of pull requests is managed by Graphite. Learn more about stacking.

[linear]

RAI-465

[0xgleb]

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Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

crates/event-sorcery/src/projection.rs (1)

131-146: ⚠️ Potential issue | 🟠 Major | ⚡ Quick win

Expose a public constructor for custom ViewBackends.

ViewBackend is now public, but the only public constructor still builds SqliteViewBackend, and the backend-agnostic new stays #[cfg(test)] pub(crate). That means downstream code can write Projection<MyEntity, MyBackend> but has no public way to instantiate it, so the new abstraction is effectively unusable outside this crate. Please either add a public backend-agnostic factory or keep ViewBackend internal until that constructor exists.

Also applies to: 438-447

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@crates/event-sorcery/src/projection.rs` around lines 131 - 146, The public
ViewBackend type lacks a public, backend-agnostic constructor for Projection:
currently only backend-specific constructors like sqlite (pub fn sqlite) exist
and the generic Projection::new is #[cfg(test)] pub(crate), so external crates
cannot instantiate Projection with custom ViewBackend implementations; add a
public constructor (e.g., pub fn new(repo: Arc<dyn
ViewBackend<Lifecycle<Entity>, Lifecycle<Entity>> + Send + Sync>, pool:
Option<SqlitePool>, table_name: Option<String>) -> Self) or make the existing
generic factory (Projection::new) pub (remove #[cfg(test)] and change visibility
to pub) so callers can create Projection<T, B> from any backend implementation,
and mirror the same change for the other backend-specific constructors
referenced around the lines noted (e.g., the similar constructor at 438-447) so
all backends are constructible externally.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Outside diff comments:
In `@crates/event-sorcery/src/projection.rs`:
- Around line 131-146: The public ViewBackend type lacks a public,
backend-agnostic constructor for Projection: currently only backend-specific
constructors like sqlite (pub fn sqlite) exist and the generic Projection::new
is #[cfg(test)] pub(crate), so external crates cannot instantiate Projection
with custom ViewBackend implementations; add a public constructor (e.g., pub fn
new(repo: Arc<dyn ViewBackend<Lifecycle<Entity>, Lifecycle<Entity>> + Send +
Sync>, pool: Option<SqlitePool>, table_name: Option<String>) -> Self) or make
the existing generic factory (Projection::new) pub (remove #[cfg(test)] and
change visibility to pub) so callers can create Projection<T, B> from any
backend implementation, and mirror the same change for the other
backend-specific constructors referenced around the lines noted (e.g., the
similar constructor at 438-447) so all backends are constructible externally.

---

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📥 Commits

Reviewing files that changed from the base of the PR and between 1670c19 and f31b57c.

📒 Files selected for processing (3)

• crates/event-sorcery/src/lib.rs
• crates/event-sorcery/src/projection.rs
• crates/event-sorcery/src/view_backend.rs

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