Added support for JSON fields (#48)

tests/postgres/test_json.cpp

@@ -0,0 +1,53 @@
+#ifndef SQLGEN_BUILD_DRY_TESTS_ONLY
+
+#include <gtest/gtest.h>
+
+#include <rfl.hpp>
+#include <rfl/json.hpp>
+#include <sqlgen.hpp>
+#include <sqlgen/postgres.hpp>
+#include <vector>
+
+namespace test_json {
+
+struct Person {
+  std::string first_name;
+  std::string last_name;
+  int age;
+  sqlgen::JSON<std::optional<std::vector<Person>>> children;
+};
+
+TEST(postgres, test_json) {
+  const auto children = std::vector<Person>(
+      {Person{.first_name = "Bart", .last_name = "Simpson", .age = 10},
+       Person{.first_name = "Lisa", .last_name = "Simpson", .age = 8},
+       Person{.first_name = "Maggie", .last_name = "Simpson", .age = 0}});
+
+  const auto homer = Person{.first_name = "Homer",
+                            .last_name = "Simpson",
+                            .age = 45,
+                            .children = children};
+
+  const auto credentials = sqlgen::postgres::Credentials{.user = "postgres",
+                                                         .password = "password",
+                                                         .host = "localhost",
+                                                         .dbname = "postgres"};
+
+  using namespace sqlgen;
+  using namespace sqlgen::literals;
+
+  const auto people2 = sqlgen::postgres::connect(credentials)
+                           .and_then(drop<Person> | if_exists)
+                           .and_then(write(std::ref(homer)))
+                           .and_then(sqlgen::read<std::vector<Person>>)
+                           .value();
+
+  const std::string expected =
+      R"([{"first_name":"Homer","last_name":"Simpson","age":45,"children":[{"first_name":"Bart","last_name":"Simpson","age":10},{"first_name":"Lisa","last_name":"Simpson","age":8},{"first_name":"Maggie","last_name":"Simpson","age":0}]}])";
+
+  EXPECT_EQ(rfl::json::write(people2), expected);
+}
+
+}  // namespace test_json
+
+#endif