diff --git a/Cargo.lock b/Cargo.lock
index 4d7f909..c66f6c6 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -137,8 +137,6 @@ dependencies = [
  "tower-http",
  "tracing",
  "utoipa",
- "utoipa-scalar",
- "utoipa-swagger-ui",
  "validator",
 ]
 
@@ -640,9 +638,11 @@ dependencies = [
  "api",
  "app",
  "axum 0.7.5",
+ "doc",
  "http-body-util",
  "models",
  "sea-orm",
+ "serde_json",
  "shuttle-axum",
  "shuttle-runtime",
  "shuttle-shared-db",
@@ -956,6 +956,18 @@ dependencies = [
  "syn 2.0.71",
 ]
 
+[[package]]
+name = "doc"
+version = "0.1.0"
+dependencies = [
+ "api",
+ "axum 0.7.5",
+ "models",
+ "utoipa",
+ "utoipa-scalar",
+ "utoipa-swagger-ui",
+]
+
 [[package]]
 name = "dotenvy"
 version = "0.15.7"
@@ -2872,9 +2884,9 @@ dependencies = [
 
 [[package]]
 name = "sea-query"
-version = "0.31.0-rc.8"
+version = "0.31.0-rc.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f62d5f33113186ad8b0f47c3522e40f9c31cdbc3056a8d7ad402e7498fc29789"
+checksum = "2ad45af4aedefa04c296d623ffb7e409a24d4bfc0b5599b43f5726d88cc7ff2a"
 dependencies = [
  "educe",
  "inherent",
diff --git a/Cargo.toml b/Cargo.toml
index a75de96..38d91f3 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -11,13 +11,14 @@ license = "MIT"
 readme = "README.md"
 
 [workspace]
-members = ["api", "app", "models", "migration", "utils"]
+members = ["api", "app", "doc", "models", "migration", "utils"]
 
 [workspace.dependencies]
 axum = { version = "0.7.5", default-features = false }
 tower = { version = "0.4.13", default-features = false }
 sea-orm = { version = "1.0.0-rc.7", default-features = false }
 serde = { version = "1", features = ["derive"] }
+serde_json = { version = "1", default-features = false }
 tracing = "0.1.40"
 utoipa = { version = "5.0.0-alpha.0", default-features = false }
 validator = { version = "0.18", default-features = false }
@@ -25,6 +26,7 @@ validator = { version = "0.18", default-features = false }
 [dependencies]
 api = { path = "api" }
 utils = { path = "utils" }
+doc = { path = "doc" }
 
 sea-orm = { workspace = true }
 
@@ -48,6 +50,7 @@ app = { path = "app" }
 models = { path = "models" }
 
 http-body-util = "0.1.2"
+serde_json = { workspace = true }
 
 [features]
 default = []
diff --git a/README.md b/README.md
index e3903ca..633cb29 100644
--- a/README.md
+++ b/README.md
@@ -17,23 +17,51 @@ You probably don't need [Rust on Rails](https://github.com/loco-rs/loco).
 
 ## Module hierarchy
 
-- `api`: Axum logic
-  - `api::routers`: Axum endpoints
-  - `api::doc`: Utoipa doc declaration
-  - `api::error`: Error handling
-  - `api::extractor` and `api::validation`: Axum extractor and JSON validation
+### API logic
+
+- `api::routers`: Axum endpoints
+- `api::error`: Models and traits for error handling
+- `api::extractor` Custom Axum extractors
+  - `api::extractor::json`: `Json` for bodies and responses
+  - `api::extractor::valid`: `Valid` for JSON body validation
+- `api::validation`: JSON validation model based on `validator`
 - `api::models`: Non domain model API models
   - `api::models::response`: JSON error response
-- `app`: DB/API-agnostic logic
-  - `app::services`: DB manipulation (CRUD) functions
-  - `app::config`: DB or API configuration
-  - `app::state`: APP state, e.g. DB connection
-- `models`: DB/API-agnostic models
-  - `models::domains`: SeaORM domain models
-  - `models::params`: Serde input parameters for creating/updating domain models in DB
-  - `models::schemas`: Serde output schemas for combining different domain models
-  - `models::queries`: Serde queries for filtering domain models
+
+### OpenAPI documentation
+
+- `doc`: Utoipa doc declaration
+
+### API-agonistic DB logic
+
+Main concept: Web framework is replaceable.
+
+All modules here should not include any specific API web framework logic.
+
+- `app::services`: DB manipulation (CRUD) functions
+- `app::config`: DB or API server configuration
+- `app::state`: APP state, e.g. DB connection
+- `app::error`: APP errors used by `api::error`. e.g. "User not found"
+
+### DB/API-agnostic domain models
+
+Main concept: Database (Sqlite/MySQL/PostgreSQL) is replaceable.
+
+Except `models::domains` and `migration`, all modules are ORM library agnostic.
+
+- `models::domains`: SeaORM domain models
+- `models::params`: Serde input parameters for creating/updating domain models in DB
+- `models::schemas`: Serde output schemas for combining different domain models
+- `models::queries`: Serde queries for filtering domain models
 - `migration`: SeaORM migration files
+
+### Unit and integration tests
+
+- `tests::api`: API integration tests. Hierarchy is the same as `api::routers`
+- `tests::app`: DB/ORM-related unit tests. Hierarchy is the same as `app::services`
+
+### Others
+
 - `utils`: Utility functions
 - `main`: Tokio and Shuttle conditional entry point
 
diff --git a/api/Cargo.toml b/api/Cargo.toml
index 5279b46..56a19de 100644
--- a/api/Cargo.toml
+++ b/api/Cargo.toml
@@ -20,14 +20,7 @@ dotenvy = "0.15.7"
 sea-orm = { workspace = true }
 
 # doc
-utoipa = { workspace = true, features = ["axum_extras"] }
-utoipa-swagger-ui = { version = "7.1.1-alpha.0", features = [
-    "axum",
-    "vendored",
-], default-features = false }
-utoipa-scalar = { version = "0.2.0-alpha.0", features = [
-    "axum",
-], default-features = false }
+utoipa = { workspace = true }
 
 # local dependencies
 app = { path = "../app" }
diff --git a/api/src/doc/mod.rs b/api/src/doc/mod.rs
deleted file mode 100644
index 7c90665..0000000
--- a/api/src/doc/mod.rs
+++ /dev/null
@@ -1,18 +0,0 @@
-use utoipa::OpenApi;
-
-mod root;
-mod user;
-
-#[derive(OpenApi)]
-#[openapi(
-    nest(
-        (path = "/", api = root::RootApi),
-        (path = "/users", api = user::UserApi),
-    ),
-    tags(
-        (name = "root", description = "Root API"),
-        (name = "user", description = "User API")
-
-    )
-)]
-pub struct ApiDoc;
diff --git a/api/src/init.rs b/api/src/init.rs
index 67c45e8..cd723a7 100644
--- a/api/src/init.rs
+++ b/api/src/init.rs
@@ -1,19 +1,14 @@
 use axum::Router;
 use sea_orm::{ConnectOptions, Database, DatabaseConnection};
-use utoipa::OpenApi;
-use utoipa_scalar::{Scalar, Servable as ScalarServable};
-use utoipa_swagger_ui::SwaggerUi;
 
 use app::config::Config;
 use app::state::AppState;
 
-use crate::doc::ApiDoc;
 use crate::routers::create_router;
 
+// TODO: middleware, logging, authentication
 pub fn setup_router(conn: DatabaseConnection) -> Router {
     create_router(AppState { conn })
-        .merge(SwaggerUi::new("/docs").url("/openapi.json", ApiDoc::openapi()))
-        .merge(Scalar::with_url("/scalar", ApiDoc::openapi()))
 }
 
 pub fn setup_config() -> Config {
diff --git a/api/src/lib.rs b/api/src/lib.rs
index ec292e4..ceba9b9 100644
--- a/api/src/lib.rs
+++ b/api/src/lib.rs
@@ -1,9 +1,9 @@
-mod doc;
 mod error;
 mod extractor;
 mod init;
-mod models;
-mod routers;
 mod validation;
 
+pub mod models;
+pub mod routers;
+
 pub use init::{setup_config, setup_db, setup_router};
diff --git a/api/src/routers/mod.rs b/api/src/routers/mod.rs
index 1d42239..e121934 100644
--- a/api/src/routers/mod.rs
+++ b/api/src/routers/mod.rs
@@ -7,7 +7,6 @@ use app::state::AppState;
 use root::create_root_router;
 use user::create_user_router;
 
-// TODO: middleware, testing, logging
 pub fn create_router(state: AppState) -> Router {
     Router::new()
         .nest("/users", create_user_router(state.clone()))
diff --git a/doc/Cargo.toml b/doc/Cargo.toml
new file mode 100644
index 0000000..943526f
--- /dev/null
+++ b/doc/Cargo.toml
@@ -0,0 +1,19 @@
+[package]
+name = "doc"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[dependencies]
+axum = { workspace = true }
+utoipa = { workspace = true, features = ["axum_extras"] }
+utoipa-swagger-ui = { version = "7.1.1-alpha.0", features = [
+    "axum",
+    "vendored",
+], default-features = false }
+utoipa-scalar = { version = "0.2.0-alpha.0", features = [
+    "axum",
+], default-features = false }
+
+api = { path = "../api" }
+models = { path = "../models" }
diff --git a/doc/src/lib.rs b/doc/src/lib.rs
new file mode 100644
index 0000000..1b9cbfe
--- /dev/null
+++ b/doc/src/lib.rs
@@ -0,0 +1,32 @@
+use axum::Router;
+use utoipa::OpenApi;
+use utoipa_scalar::{Scalar, Servable as ScalarServable};
+use utoipa_swagger_ui::SwaggerUi;
+
+mod root;
+mod user;
+
+#[derive(OpenApi)]
+#[openapi(
+    nest(
+        (path = "/", api = root::RootApi),
+        (path = "/users", api = user::UserApi),
+    ),
+    tags(
+        (name = "root", description = "Root API"),
+        (name = "user", description = "User API")
+
+    )
+)]
+struct _ApiDoc;
+
+pub trait ApiDoc {
+    fn attach_doc(self) -> Self;
+}
+
+impl ApiDoc for Router {
+    fn attach_doc(self) -> Self {
+        self.merge(SwaggerUi::new("/docs").url("/openapi.json", _ApiDoc::openapi()))
+            .merge(Scalar::with_url("/scalar", _ApiDoc::openapi()))
+    }
+}
diff --git a/api/src/doc/root.rs b/doc/src/root.rs
similarity index 55%
rename from api/src/doc/root.rs
rename to doc/src/root.rs
index 787faee..c75de83 100644
--- a/api/src/doc/root.rs
+++ b/doc/src/root.rs
@@ -1,7 +1,7 @@
 use utoipa::OpenApi;
 
-use crate::routers::root::*;
+use api::routers::root::*;
 
 #[derive(OpenApi)]
 #[openapi(paths(root))]
-pub struct RootApi;
+pub(super) struct RootApi;
diff --git a/api/src/doc/user.rs b/doc/src/user.rs
similarity index 76%
rename from api/src/doc/user.rs
rename to doc/src/user.rs
index 02c3010..bb9d9e4 100644
--- a/api/src/doc/user.rs
+++ b/doc/src/user.rs
@@ -3,8 +3,8 @@ use utoipa::OpenApi;
 use models::params::user::CreateUserParams;
 use models::schemas::user::{UserListSchema, UserSchema};
 
-use crate::models::{ApiErrorResponse, ParamsErrorResponse};
-use crate::routers::user::*;
+use api::models::{ApiErrorResponse, ParamsErrorResponse};
+use api::routers::user::*;
 
 #[derive(OpenApi)]
 #[openapi(
@@ -17,4 +17,4 @@ use crate::routers::user::*;
         ParamsErrorResponse,
     ))
 )]
-pub struct UserApi;
+pub(super) struct UserApi;
diff --git a/models/Cargo.toml b/models/Cargo.toml
index 559344b..0fa6b0d 100644
--- a/models/Cargo.toml
+++ b/models/Cargo.toml
@@ -7,7 +7,7 @@ publish = false
 
 [dependencies]
 serde = { workspace = true }
-serde_json = "1"
+serde_json = { workspace = true }
 sea-orm = { workspace = true, features = [
   "sqlx-postgres",
   "sqlx-sqlite",
diff --git a/src/main.rs b/src/main.rs
index 8fc0908..9847571 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,3 +1,5 @@
+use api::{setup_db, setup_router};
+use doc::ApiDoc;
 use utils::migrate;
 
 #[cfg(not(feature = "shuttle"))]
@@ -19,10 +21,10 @@ async fn main() {
     let config = api::setup_config();
     create_dev_db(&config.db_url);
 
-    let conn = api::setup_db(&config.db_url).await;
+    let conn = setup_db(&config.db_url).await;
     migrate(&conn).await.expect("Migration failed!");
 
-    let router = api::setup_router(conn);
+    let router = setup_router(conn).attach_doc();
     let listener = tokio::net::TcpListener::bind(&config.get_server_url())
         .await
         .unwrap();
@@ -36,9 +38,9 @@ async fn main() {
 async fn main(#[shuttle_shared_db::Postgres] db_url: String) -> shuttle_axum::ShuttleAxum {
     tracing::info!("Starting with shuttle");
 
-    let conn = api::setup_db(&db_url).await;
+    let conn = setup_db(&db_url).await;
     migrate(&conn).await.expect("Migration failed!");
 
-    let router = api::setup_router(conn);
+    let router = setup_router(conn).attach_doc();
     Ok(router.into())
 }