pqxx 和 null 外键

Question

I have the following code:

void DB_Series_Base::doInsert(pqxx::connection &conn, Series &obj) {
    pqxx::work work {conn};
    string sql { string{"INSERT INTO series ("} + INSERT_LIST + ") VALUES ($1, $2) RETURNING id" };
    pqxx::result results = work.exec_params(sql,
        obj.getName().length() > 0 ? obj.getName().c_str() : nullptr,
        obj.getUniverseId() != 0 ? obj.getUniverseId(): nullptr);
    work.commit();
    obj.setId(results[0][0].as<int>());
}

This does not compile with the error 'non-pointer operand type 'int' incompatible with nullptr'. Okay, I get it. The trinary operator needs to return a value with a consistent datatype.

In this place, universeId is a foreign key into the Universe table.

I tried this:

void DB_Series_Base::doInsert(pqxx::connection &conn, Series &obj) {
    int universeId = obj.getUniverseId();

    pqxx::work work {conn};
    string sql { string{"INSERT INTO series ("} + INSERT_LIST + ") VALUES ($1, $2) RETURNING id" };
    pqxx::result results = work.exec_params(sql,
        obj.getName().length() > 0 ? obj.getName().c_str() : nullptr,
        &universeId);
    work.commit();
    obj.setId(results[0][0].as<int>());
}

But this generates link errors (which I don't usually have):

Undefined symbols for architecture x86_64:
  "pqxx::string_traits<int*>::size_buffer(int* const&)", referenced from:
      unsigned long pqxx::size_buffer<int*>(int* const&) in libauthor-mac.a(DB_Series_Base.o)
  "pqxx::string_traits<int*>::into_buf(char*, char*, int* const&)", referenced from:
      std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > pqxx::to_string<int*>(int* const&) in libauthor-mac.a(DB_Series_Base.o)
  "pqxx::nullness<int*, void>::is_null(int* const&)", referenced from:
      bool pqxx::is_null<int*>(int* const&) in libauthor-mac.a(DB_Series_Base.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [Makefile:39: AuthorServer] Error 1```

The only other thing I can think of is to entirely build custom SQL for each insert / update, and embed "NULL" in the VALUES section instead of $2 etc. That's kind of gross, but I can do it if no one has a better suggestion.

So... how can I pass either the actual foreign key (universeId) -or- nullptr as apprpriate?

Answer 1

Oh, this is cool. I discovered the nullif function.

void
TestDatabase::testNullForeignKeys() {
    connect();
    string sqlSeries { "INSERT INTO series(name, universe_id) VALUES ($1, nullif($2, 0)) RETURNING id" };

    int universeId = 0;

    pqxx::work populateSeries {*connection};
    pqxx::result results = populateSeries.exec_params(sqlSeries, "Test Series", universeId);
    populateSeries.commit();
}

Notice the nullif($2,0) in the VALUES portion of the INSERT statement. What this says:

If the first argument == the second one, return null. Otherwise return the first argument. So if universeId is 0, nullif returns NULL. Otherwise it returns the real value.

This solves my problem.