Add sqlx flatten support

README.md

@@ -246,6 +246,39 @@ struct Submodel {
 }
 ```
 
+### Flatten support
+
+Atmosphere supports flattening nested structs into the parent table using `#[sql(flatten)]`.
+This generates columns for the nested type at compile time and binds them on INSERT/UPDATE.
+
+```rust
+use atmosphere::prelude::*;
+
+#[derive(FlattenFields)]
+struct ContactInfo {
+    email: String,
+    phone: Option<String>,
+}
+
+#[table(schema = "public", name = "user")]
+struct User {
+    #[sql(pk)]
+    id: i32,
+    name: String,
+    #[sql(flatten, prefix)]
+    contact: ContactInfo,
+}
+```
+
+Prefix modes:
+- `#[sql(flatten)]` no prefix (columns must be unique)
+- `#[sql(flatten, prefix)]` or `#[sql(flatten, prefix = auto)]` auto prefix with field name
+- `#[sql(flatten, prefix = "c_")]` custom prefix
+
+Notes:
+- Changing prefix requires a database migration (column names change).
+- Recommended max nesting depth: 2 levels (Postgres identifier limit is 63 chars).
+
 ## Contribution
 
 We welcome contributions! Please see [our contribution guidelines](CONTRIBUTING.md) for more details.