Deprecate introspection of incomplete SQLite schema

[morozov]

Q	A
Type	deprecation

Background

The SQL syntax allows omitting referenced column names in a foreign key constraint declaration. In this case, it's implied that the constraint references the primary key columns. Normally, databases (e.g. Postgres) present the resulting definition of the constraint in the introspection schema (i.e. the actual column names) however, SQLite reports the original SQL statement.

A foreign key constraint declaration that was declared with omitted column names is reported as not referencing any columns. And if the referenced table doesn't exist (which SQLite allows), we get a foreign key without columns.

This is a very corner case that shouldn't happen normally. However, if it does, DBAL should throw an exception instead of producing invalid results.

Problem statement

From the data model standpoint, a foreign key constraint definition without columns doesn't make sense and will be impossible to construct in 5.0.

Refactoring

Additionally, the introspection code has been refactored. The current implementation reads as "if the to column contains null, then introspect the referenced table PK and add its columns to the existing columns". While this is technically correct, the "add to the existing columns" case will never happen because there will be only one row representing a foreign key without columns. Instead, we introspect all keys and then fetch the PKs for the ones that don't have columns. This is easier to comprehend and easier to put the deprecation logic to.

[greg0ire]

however, SQL reports

Did you mean to write "SQLite" here?

[morozov]

however, SQL reports

Did you mean to write "SQLite" here?

Yes, fixed. Thanks!