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Restoring a SQLite backup into PostgreSQL died part-way with
insert or update on table "library_files" violates foreign key
constraint "library_files_folder_id_fkey"
DETAIL: Key (folder_id)=(1) is not present in table "library_folders".
The import recreates the schema and is supposed to create every table
without foreign keys, so the order rows arrive in cannot matter; the
constraints are added back once the data has landed. Phase 1 did that by
discarding each ForeignKeyConstraint from table.constraints before
create_all -- which only suppresses the inline REFERENCES clause.
Table.foreign_key_constraints is derived from the columns' ForeignKey
objects and was never touched, and when create_all meets a dependency
cycle it cannot sort, it falls back to emitting those tables' keys as
separate ALTER TABLE ... ADD FOREIGN KEY statements read from exactly
that property.
library_files, library_folders and print_archives form such a cycle, so
twelve constraints survived across the three of them -- measured against
a real PostgreSQL by running the old phase verbatim. The same cycle also
costs those tables their place in sorted_tables, so they were imported
alphabetically, putting library_files ahead of the library_folders rows
its folder_id references.
Phase 1 now creates the tables normally and drops every foreign key from
pg_constraint afterwards, in the same transaction, scoped to contype 'f'
in the public schema. That is indifferent to how create_all chose to
emit them, so a future cycle between other tables cannot bring this
back. Phase 3 is unchanged.
This also removes a second fault: the keys were stripped from the
process-wide Base.metadata and only restored after the drop/create
transaction, so a failure in between left the running app without them
until restart. The metadata is no longer modified at all.
Verified end to end against a real PostgreSQL -- a backup whose child
rows import before their parents restores cleanly, with all 90
constraints back afterwards. Four regression tests added.