Added support for joins (#25)

tests/sqlite/test_joins_nested_grouped.cpp

@@ -0,0 +1,80 @@
+#include <gtest/gtest.h>
+
+#include <rfl.hpp>
+#include <rfl/json.hpp>
+#include <sqlgen.hpp>
+#include <sqlgen/sqlite.hpp>
+#include <vector>
+
+namespace test_joins_nested_grouped {
+
+struct Person {
+  sqlgen::PrimaryKey<uint32_t> id;
+  std::string first_name;
+  std::string last_name;
+  double age;
+};
+
+struct Relationship {
+  uint32_t parent_id;
+  uint32_t child_id;
+};
+
+TEST(sqlite, test_joins_nested_grouped) {
+  const auto people1 = std::vector<Person>(
+      {Person{
+           .id = 0, .first_name = "Homer", .last_name = "Simpson", .age = 45},
+       Person{
+           .id = 1, .first_name = "Marge", .last_name = "Simpson", .age = 40},
+       Person{.id = 2, .first_name = "Bart", .last_name = "Simpson", .age = 10},
+       Person{.id = 3, .first_name = "Lisa", .last_name = "Simpson", .age = 8},
+       Person{
+           .id = 4, .first_name = "Maggie", .last_name = "Simpson", .age = 0}});
+
+  const auto relationships =
+      std::vector<Relationship>({Relationship{.parent_id = 0, .child_id = 2},
+                                 Relationship{.parent_id = 0, .child_id = 3},
+                                 Relationship{.parent_id = 0, .child_id = 4},
+                                 Relationship{.parent_id = 1, .child_id = 2},
+                                 Relationship{.parent_id = 1, .child_id = 3},
+                                 Relationship{.parent_id = 1, .child_id = 4}});
+
+  using namespace sqlgen;
+
+  struct ParentAndChild {
+    std::string last_name;
+    std::string first_name_child;
+    double avg_parent_age_at_birth;
+  };
+
+  const auto get_children =
+      select_from<Relationship, "t1">("parent_id"_t1 | as<"id">,
+                                      "first_name"_t2 | as<"first_name">,
+                                      "age"_t2 | as<"age">) |
+      left_join<Person, "t2">("id"_t2 == "child_id"_t1);
+
+  const auto get_people =
+      select_from<Person, "t1">(
+          "last_name"_t1 | as<"last_name">,
+          "first_name"_t2 | as<"first_name_child">,
+          avg("age"_t1 - "age"_t2) | as<"avg_parent_age_at_birth">) |
+      inner_join<"t2">(get_children, "id"_t1 == "id"_t2) |
+      group_by("last_name"_t1, "first_name"_t2) | order_by("first_name"_t2) |
+      to<std::vector<ParentAndChild>>;
+
+  const auto people = sqlite::connect()
+                          .and_then(write(std::ref(people1)))
+                          .and_then(write(std::ref(relationships)))
+                          .and_then(get_people)
+                          .value();
+
+  const std::string expected_query =
+      R"(SELECT t1."last_name" AS "last_name", t2."first_name" AS "first_name_child", AVG((t1."age") - (t2."age")) AS "avg_parent_age_at_birth" FROM "Person" t1 INNER JOIN (SELECT t1."parent_id" AS "id", t2."first_name" AS "first_name", t2."age" AS "age" FROM "Relationship" t1 LEFT JOIN "Person" t2 ON t2."id" = t1."child_id") t2 ON t1."id" = t2."id" GROUP BY t1."last_name", t2."first_name" ORDER BY t2."first_name")";
+
+  const std::string expected =
+      R"([{"last_name":"Simpson","first_name_child":"Bart","avg_parent_age_at_birth":32.5},{"last_name":"Simpson","first_name_child":"Lisa","avg_parent_age_at_birth":34.5},{"last_name":"Simpson","first_name_child":"Maggie","avg_parent_age_at_birth":42.5}])";
+  EXPECT_EQ(sqlite::to_sql(get_people), expected_query);
+  EXPECT_EQ(rfl::json::write(people), expected);
+}
+
+}  // namespace test_joins_nested_grouped