feat: add column_attr::primary_key for inline single-column primary keys

Allows declaring PRIMARY KEY directly on a column field instead of
requiring a separate table_constraints<T> specialization.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Pierre-Luc Gagné

CHANGELOG.md

@@ -10,6 +10,7 @@ Versioning follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### Added
 
+- `column_attr::primary_key` — inline column-level `PRIMARY KEY` attribute for single-column primary keys, e.g. `COLUMN_FIELD(id, uint32_t, column_attr::primary_key, column_attr::auto_increment)`
 - `date_type` for MySQL `DATE` columns — stores `std::chrono::sys_days`, supports serialization (`'YYYY-MM-DD'`), deserialization, and `std::optional<date_type>` for nullable columns
 
 ---
@@ -18,7 +19,7 @@ Versioning follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### Removed
 
-- `table_inline_primary_key<T>` trait and automatic `PRIMARY KEY AUTO_INCREMENT` on the first column — define primary keys explicitly via `table_constraints<T>` or column attributes (`column_attr::auto_increment`)
+- `table_inline_primary_key<T>` trait and automatic `PRIMARY KEY AUTO_INCREMENT` on the first column — define primary keys explicitly via `column_attr::primary_key`, `table_constraints<T>`, or both
 
 ---
 

CLAUDE.md

@@ -53,7 +53,7 @@ Style: Google-based, 120-char line limit, 4-space indent (`.clang-format` at roo
 - `sql_dql.hpp` — `select(col1{}, col2{})`, `count(T{})`, `describe(T{})`
 - `connect_options.hpp` — `connect_options` fluent builder and `ssl_mode` enum for pre-connect `mysql_options()` configuration (timeouts, SSL/TLS, charset, compression, etc.)
 - `charset_name.hpp` — `charset_name` strong type for character set names
-- `column_field.hpp` / `column_field_base_*.hpp` — `column_field<"name", Type>` descriptors and `COLUMN_FIELD(name, type)` macro
+- `column_field.hpp` / `column_field_base_*.hpp` — `column_field<"name", Type, Attrs...>` descriptors, `COLUMN_FIELD(name, type, attrs...)` macro, and column attributes (`column_attr::primary_key`, `auto_increment`, `unique`, `default_current_timestamp`, `on_update_current_timestamp`, `comment<"...">`, `collate<"...">`)
 - `schema_generator.hpp` — derives CREATE TABLE SQL from a C++ struct at compile time using Boost.PFR
 - `sql_varchar.hpp`, `sql_numeric.hpp`, `sql_temporal.hpp` — library-specific SQL types (`varchar_type<N>`, `decimal_type<P,S>`, `datetime_type<FSP>`, etc.)
 - `metadata.hpp` — types for querying `information_schema`

lib/include/ds_mysql/column_field.hpp

@@ -42,6 +42,7 @@ struct column_field : column_field_detail::base<T> {
     using column_field_detail::base<T>::base;
     using column_field_detail::base<T>::operator=;
 
+    static constexpr bool ddl_primary_key = (std::same_as<Attrs, column_attr::primary_key> || ...);
     static constexpr bool ddl_auto_increment = (std::same_as<Attrs, column_attr::auto_increment> || ...);
     static constexpr bool ddl_unique = (std::same_as<Attrs, column_attr::unique> || ...);
     static constexpr bool ddl_default_current_timestamp =

lib/include/ds_mysql/column_field_base_core.hpp

@@ -15,6 +15,7 @@ struct column_field_tag {};
 
 namespace column_attr {
 
+struct primary_key {};
 struct auto_increment {};
 struct unique {};
 struct default_current_timestamp {};

lib/include/ds_mysql/sql_ddl.hpp

@@ -1024,6 +1024,9 @@ template <typename T, std::size_t I>
     if constexpr (!is_field_nullable_v<field_type>) {
         col += " NOT NULL";
     }
+    if constexpr (requires { field_type::ddl_primary_key; } && field_type::ddl_primary_key) {
+        col += " PRIMARY KEY";
+    }
     if constexpr (requires { field_type::ddl_auto_increment; } && field_type::ddl_auto_increment) {
         col += " AUTO_INCREMENT";
     }

tests/integration/mysql/test_query_builder.cpp

@@ -20,7 +20,7 @@ namespace ds_mysql {
 namespace {
 
 struct trade {
-    COLUMN_FIELD(id, uint32_t)
+    COLUMN_FIELD(id, uint32_t, column_attr::primary_key, column_attr::auto_increment)
     COLUMN_FIELD(account_id, std::optional<uint32_t>)
     COLUMN_FIELD(code, varchar_type<32>)
     COLUMN_FIELD(type, varchar_type<64>)
@@ -1449,7 +1449,7 @@ suite<"Typed Query Column Count Coverage"> typed_query_col_count_suite = [] {
 // ===================================================================
 
 struct auto_inc_row {
-    COLUMN_FIELD(id, uint32_t, column_attr::auto_increment)
+    COLUMN_FIELD(id, uint32_t, column_attr::primary_key, column_attr::auto_increment)
     COLUMN_FIELD(label, varchar_type<64>)
 };
 

tests/unit/test_ddl.cpp

@@ -196,6 +196,11 @@ struct table_with_attrs {
     COLUMN_FIELD(id, uint32_t)
     COLUMN_FIELD(name, varchar_type<255>)
 };
+
+struct table_with_pk {
+    COLUMN_FIELD(id, uint32_t, column_attr::primary_key, column_attr::auto_increment)
+    COLUMN_FIELD(name, varchar_type<255>)
+};
 }  // namespace
 
 template <>
@@ -800,6 +805,16 @@ suite<"DDL"> ddl_suite = [] {
         expect(!sql.contains("AUTO_INCREMENT")) << sql;
     };
 
+    "create_table - column_attr::primary_key emits PRIMARY KEY on column"_test = [] {
+        auto const sql = create_table(table_with_pk{}).build_sql();
+        expect(sql ==
+               "CREATE TABLE table_with_pk (\n"
+               "    id INT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,\n"
+               "    name VARCHAR(255) NOT NULL\n"
+               ");\n"s)
+            << sql;
+    };
+
     "create_table with table_attributes trait - emits default ENGINE and CHARSET"_test = [] {
         auto const sql = create_table(table_with_attrs{}).build_sql();
         expect(sql ==