Building a DDD Data Runtime with Generated Typed Queries in Rust

TeaQL is a data layer for applications where the domain model is the center of the system rather than a thin mapping over tables. The current Rust implementation carries over ideas from the Java TeaQL stack, but with a smaller scope: PostgreSQL, SQLite, a Rust-native query AST, generated typed APIs, and no web framework dependency.

The goal is direct: instead of writing most application data access as handmade repository methods or raw SQL fragments, a domain model can generate a Rust crate with entity types, relation metadata, query builders, checker hooks, behavior hooks, and graph-save entrypoints.

Application code then works with a high-level API:

let platforms = Q::platforms()
    .select_merchant_list_with(
        Q::merchants()
            .select_name()
            .which_names_contain("TeaQL"),
    )
    .execute_for_list(&ctx)
    .await?;

This is not meant to replace every way of using SQL from Rust. If a service is mostly carefully tuned SQL, direct sqlx is probably a better fit. If the preferred abstraction is an ORM with a large Rust ecosystem, Diesel or SeaORM will be more familiar. TeaQL is aimed at a different case: large domain models where repeated relation loading, graph persistence, validation, and statistics queries become their own layer of application logic.

Why Generate the API?

The original pressure came from systems where the same entity model needed to support several kinds of behavior:

• ordinary list and detail queries;
• nested relation loading, including paths such as merchant.platform or platform.merchant_list;
• graph writes where a parent object and children are committed together;
• additive schema bootstrap for development and tests;
• checker and validation logic that can inspect and fix entities;
• simple and grouped statistics;
• JSON serialization and JSON-expression style search.

You can build all of that by hand, but the code tends to become repetitive in two places. First, relation names and field names are repeated across query methods, repository code, and validation code. Second, application developers end up switching between typed domain objects and untyped row maps. TeaQL tries to keep the generated surface typed while letting the runtime keep a generic query and graph model internally.

For example, a generated service crate exposes Q::platforms() and Q::merchants() rather than asking application code to construct SelectQuery::new("Platform") directly. Low-level query objects still exist, but they are not the normal application-level API.

Runtime Pieces

The Rust workspace is split into small crates:

• teaql-core: values, records, entity descriptors, query AST, expressions, commands, and SmartList<T>;
• teaql-sql: SQL compilation and dialect-neutral compiled query types;
• teaql-runtime: UserContext, repository resolution, behavior hooks, checker hooks, graph writes, relation enhancement, events, and optional SQLx executors;
• teaql-macros: #[derive(TeaqlEntity)] for descriptors and typed record/entity mapping;
• teaql-provider-sqlx-postgres, teaql-provider-sqlx-sqlite, teaql-provider-sqlx-mysql, and teaql-provider-rusqlite: provider adapters and dialect-specific execution paths.

Generated crates sit above those runtime crates. A generated CRM or ERP service, for example, exports entities such as Platform and Merchant, a Q query facade, behavior skeletons, checker skeletons, repository registration, and runtime module assembly helpers.

Query Construction

TeaQL has a generic query AST, but generated code provides a domain-specific facade. Instead of this in application code:

let query = SelectQuery::new("Merchant")
    .project("id")
    .project("name")
    .filter(Expr::contains("name", "tea"));

the generated API can expose:

let merchants = Q::merchants()
    .select_name()
    .which_names_contain("tea")
    .order_by_create_time_desc()
    .page(1, 20)
    .execute_for_list(&ctx)
    .await?;

Relation loading uses the same style:

let platforms = Q::platforms()
    .select_merchant_list_with(
        Q::merchants()
            .select_name()
            .select_platform(),
    )
    .execute_for_list(&ctx)
    .await?;

One implementation detail mattered here: when a child is attached to a parent relation list, the reverse object relation should be populated too. In the example above, each merchant in platform.merchant_list should have its platform relation set. Otherwise, the result is typed but not really a domain object graph.

Graph Writes

TeaQL has a save_graph path for committing complex objects. In generated crates, application code can call a typed save helper:

merchant
    .update_name("TeaQL Merchant")
    .update_platform_id(1_u64)
    .save(&ctx)
    .await?;

Internally, the runtime turns that object into a graph plan. The plan classifies nodes by entity and operation: create, update, delete/remove, or reference. It then batches compatible work where possible and runs the graph write inside a transactional executor.

The interesting part is not only inserting children. Updating a graph means answering questions like:

• is this child new, already present, a reference, or explicitly removed?
• should missing children be soft-deleted or left alone?
• should a relation write attach a foreign key, or is it detached?
• if a reference points at a deleted row or the wrong version, should the graph write fail?

The Rust runtime currently supports nested create/update graph writes, reference-only nodes, explicit remove nodes, keep-missing relation metadata, duplicate child-id rejection, and transaction rollback for SQLite and PostgreSQL SQLx executors.

Schema Bootstrap

For local development and generated-service tests, TeaQL can bootstrap a schema from entity descriptors:

ctx.ensure_sqlite_schema().await?;

The current scope is intentionally conservative. It creates missing tables and adds missing columns. It does not try to be a destructive migration tool: no column drops, no primary-key rebuilds, and no automatic type rewrites.

That line is important because generated domain models change frequently. The bootstrap path should be safe enough for local and CI use, not pretend to replace a real production migration process.

Checkers and Domain Validation

A checker is not just a validator that rejects a row. It can inspect an object, add structured check results, and sometimes fix fields before persistence.

The generated Rust checker support lets application code write typed checker logic instead of manually reading from a Record:

impl MerchantCheckerLogic for MerchantNameChecker {
    fn check_and_fix_merchant(
        &self,
        _ctx: &UserContext,
        entity: &mut Merchant,
        status: CheckObjectStatus,
        location: &ObjectLocation,
        results: &mut CheckResults,
    ) {
        if status.is_create() {
            self.required_text(&entity.name(), "name", location, results);
        }

        self.min_string_length(&entity.name(), "name", 3, location, results);

        if entity.name() == "fix" {
            entity.update_name("fixed");
        }
    }
}

The runtime still stores the common checker interface at the record level, but the generated adapter maps records into typed entities before calling the checker. That keeps the public application code close to the domain model while preserving a generic runtime path.

Statistics

TeaQL queries can carry aggregate projections and relation aggregate metadata. The current runtime supports simple aggregates, grouped aggregates, Decimal results for SQL aggregate output, relation count/statistic attachment, and database-column-to-entity-property mapping for relation aggregate keys.

The generated Q APIs can express both simple statistics and relation statistics. For example, a service can count child rows from a parent query without asking application code to hand-build the join every time.

This area is useful but still evolving. The runtime has working SQL and memory paths for the core cases, while broader Java parity still needs more work around memory subqueries and richer relation aggregate shapes.

Tradeoffs

The most obvious tradeoff is generated code. TeaQL generates a lot of Rust. That cost shows up as compile time, larger diffs, and the need to keep templates disciplined. The benefit is that application code gets a stable, typed facade over a large domain model.

Another tradeoff is that the runtime is not purely compile-time checked. The generated APIs are typed, but the runtime still has a generic query AST, record model, and descriptor registry. That gives it flexibility for dynamic projections, aggregate rows, JSON-style search, and graph planning, but it means some mistakes are caught by generated crate tests rather than by Rust types alone.

The final tradeoff is scope. The Rust rewrite is not trying to clone every Java TeaQL feature or support every database. PostgreSQL and SQLite are enough for now. Web rendering, GraphQL integration, and broad database dialect support are outside the current Rust scope.

What Works Today

The current Rust runtime and generated crate tests cover:

• SQLite schema bootstrap and additive column changes;
• PostgreSQL schema bootstrap with SQLx;
• CRUD, optimistic locking, soft delete, and recover;
• typed entity fetch into SmartList<T>;
• nested relation enhancement;
• complex object commit through graph writes;
• transaction rollback for graph writes;
• generated Q APIs against SQLite;
• typed checker adapters from generated crates;
• JSON serialization and JSON-expression search paths;
• simple aggregates, grouped aggregates, and relation aggregate statistics.

The public examples can be run with:

cargo run -p teaql-examples --bin sqlite_schema_crud
cargo run -p teaql-examples --bin sqlite_relations_graph

The first command shows schema bootstrap and CRUD against in-memory SQLite. The second saves an object graph and reloads nested relations.

What Is Not Done

The biggest gaps are:

• more complete memory repository parity for relation enhancement and subquery execution;
• richer checker semantics, especially nested typed object locations and domain-specific labels;
• richer event payloads with old/new values and typed snapshots;
• more value types such as UUID and bytes;
• a decision on whether Rust needs a higher-level service layer above the repository/runtime APIs.

Those gaps are real. They are better kept visible than hidden behind a larger feature list.

Why This Shape Is Worth Exploring

Most Rust database libraries are good at one of two layers: explicit SQL, or a database-centric ORM. TeaQL explores a third shape: generated domain APIs over a generic runtime that understands entity graphs, relation enhancement, validation, and statistics.

That shape will not fit every codebase. It is most useful when the model is large enough that the generated API becomes an asset, and when the team wants the same domain semantics to appear in queries, graph writes, checkers, and schema bootstrap.