Support for serials as C++ data types

[MeanSquaredError]

Obviously support for serials as C++ data types has crept into ddl2cpp and sqlpp23 with the following two pieces of code

sqlpp23/include/sqlpp23/core/type_traits/data_type.h

Line 80 in 474971d

using serial = integral;

and

sqlpp23/scripts/sqlpp23-ddl2cpp

Line 418 in 474971d

if col_def.cpp_type and col_def.cpp_type.endswith("sqlpp::serial"):

IMHO this is wrong.

Firstly, all the serial pseudotypes (BIGSERIAL, SMALLSERIAL, SERIAL, SERIAL2, SERIAL4, SERIAL8) are PostgreSQL-specific extension which PostgreSQL documentation doesn't call real types. From the PostgreSQL documentation:

The data types serial and bigserial are not true types, but merely a notational convenience for setting up unique identifier columns (similar to the AUTO_INCREMENT property supported by some other databases). In the current implementation, specifying

CREATE TABLE tablename (
    colname SERIAL
);

is equivalent to specifying:

CREATE SEQUENCE tablename_colname_seq;
CREATE TABLE tablename (
    colname integer DEFAULT nextval('tablename_colname_seq') NOT NULL
);

So these are not real types even for PostgreSQL, which the moment it sees a "serial" column, essentially turns it into an integer, non-nullable column with a default, auto-increment value.

Secondly, sqlpp23 doesn't have a proper C++ data type to represent a "serial" column. Making sqlpp::serial an alias of sqlpp::integral is wrong. Instead "serial" columns should be treated as integral, non-nullable columns with a default values, just like they have been treated so far.

Thirdly, recognizing a column annotation cpp_type: sqlpp::serial is plain wrong, because there should be no sqlpp::serial in the first place. If someone wants that badly to be able to express the "serial" property through annotations, then there are two possible approaches.

• Add support for expressing the nullable and has_default properties through annotations too. Then instead of using the wrong

-- cpp_type: sqlpp::serial
ustom_type mycol,

one could write the more correct

-- cpp_type: sqlpp::integral, nullable: true, has_default: true
custom_type mycol,

• Or add a special shorthand annotation which combines the three properties into one:

-- serial
custom_type mycol,

The two approaches could be combined, one could add all three annotation properties nullable, has_default and serial (as a shorthand annotation). Just don't add sqlpp::serial as an alias of sqlpp::integral, because it is wrong and misleading.