Fix EF Core schema mapping for IsRowVersion()/xmin (#290) and ComplexProperty/ComplexCollection ToJson (#291)

[jeremydmiller]

Two related gaps in Weasel.EntityFrameworkCore's metadata → table mapping (DbContextExtensions.MapToTable), both reported on EF Core 9/10 + Npgsql. Closes #290 and #291.

#290 — Npgsql IsRowVersion() → xmin

The Npgsql-recommended optimistic-concurrency idiom maps a uint token (Property(x => x.Version).IsRowVersion()) to PostgreSQL's implicit xmin system column, not a new column. The mapper emitted xmin as a real column, so migration failed:

42701: column name "xmin" conflicts with a system column name

Fix: new provider hook Migrator.IsSystemColumn(string) (default false), overridden in PostgresqlMigrator to recognise PG's reserved system columns — tableoid, xmin, cmin, xmax, cmax, ctid. (oid is intentionally excluded — it's a valid user column on PG 12+.) MapToTable's column loop skips any property whose resolved column name is a system column.

#291 — ComplexProperty(...).ToJson() / ComplexCollection(...).ToJson()

OwnsOne(...).ToJson() worked (PR #233) because owned entities surface as navigations to a JSON-mapped entity type. EF Core 10's complex properties/collections are not navigations and have no separate entity type — their JSON container lives on the IComplexType — so they were silently skipped and the first insert failed with 42703: column "X" does not exist.

Fix: new mapComplexJsonColumns iterating entityType.GetComplexProperties(), emitting one container column per top-level JSON-mapped complex type (GetContainerColumnName/GetContainerColumnType, default jsonb; nullability from IComplexProperty.IsNullable). Both the single and collection shapes serialize into one container column. Gated #if NET10_0_OR_GREATER — complex-type JSON mapping is an EF Core 10 feature, so the net9.0 target is unaffected.

Both fixes stay in the provider-neutral mapper + the Migrator abstraction; no PG-specific reference leaks into Weasel.EntityFrameworkCore.

Answering the issues' questions

• EF Core IsRowVersion() with Npgsql fails schema migration: "xmin" conflicts with a system column name #290 "is IsRowVersion() supported?" — yes, now; it's the standard PG concurrency idiom and Weasel skips the xmin mapping correctly.
• EF Core ComplexProperty/ComplexCollection ToJson() columns missing from Weasel migration #291 "are ComplexProperty/ComplexCollection.ToJson() supported, or is OwnsOne().ToJson() canonical?" — they're supported now (EF 10); OwnsOne continues to work too.

Test plan

• PostgresqlMigratorSystemColumnTests — 11 assertions on IsSystemColumn (system columns incl. case-insensitive; ordinary columns incl. oid)
• row_version_system_column — confirms Npgsql maps Version→xmin, then that MapToTable omits xmin while keeping id/name
• complex_json_columns (net10.0) — ComplexProperty/ComplexCollection .ToJson() each produce a jsonb container column
• Existing OwnsOne().ToJson() mapping tests unchanged (2/2)
• Model-mapping tests build the EF model offline (no live DB); ran green against PG locally
• Weasel.Core / PostgreSQL / EFCore build clean; AOT smoke gate green

🤖 Generated with Claude Code

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Closes #290.
Closes #291.