fix(constraints): emit column and model constraints instead of dropping them

[lll86789]

Resolves #579

Constraints declared in a contract-enforced model's yaml never reached the database - only not_null was ever emitted. There are two independent breaks. render_column_constraint returned an empty string for every type but not_null, and sqlserver__build_model_constraints, which that method's docstring names as the place the other types are applied instead, has no call site anywhere in the repo, so model-level constraints were dropped outright. Each half reads as though the other one handles it.

Column-level constraints now render inline in the CREATE TABLE column list, and model-level constraints render there too when they carry no name:. A model-level constraint with a name: is applied by ALTER TABLE ADD CONSTRAINT once the build has swapped the new table into place and dropped the old one, which is the first point at which that name is free to reuse. SQL Server scopes constraint names per schema - unlike index names, which are scoped per table, which is what lets the existing deterministic dbt_idx_ naming work - so a name emitted inline collides with the table being replaced (Msg 2714) on every rebuild after the first. Keeping unnamed constraints inline is also worth something on its own: they are validated as the new table is built, so a violation fails the run before the swap and leaves the old table intact. A name: on a column-level constraint is ignored with a warning pointing at the model-level form.

PRIMARY KEY and UNIQUE render as NONCLUSTERED. SQL Server defaults an unqualified PRIMARY KEY to CLUSTERED, which cannot coexist with the clustered columnstore index built for as_columnstore (the default), so nonclustered is the safe default here. dbt's own expression field is the override - a constraint declaring expression: clustered keeps what it asked for - so no adapter-specific yaml key is needed. Anything else in that position is rejected at compile time rather than emitted as DDL that cannot parse.

Foreign keys now accept the to: / to_columns: form as well as the older free-text expression: form. Only the latter was matched before, so even a wired-up model constraint using to: - the form dbt-core actually produces from to: ref(...) - would have been silently discarded. A foreign key that names no target at all now warns instead of vanishing. Separately, column-level CHECK constraints are hoisted out of the column definition into table-level clauses in the same CREATE TABLE: SQL Server accepts only one column-level CHECK per column ("More than one column CHECK constraint specified for column ..."), while the table-level form has no such limit. Both are anonymous and both live in the same statement.

Each ALTER TABLE ADD CONSTRAINT is guarded on the name already being present on that table - sys.objects, matched on parent_object_id as well as name, so a same-named constraint on a sibling table cannot mask it - and the whole set is emitted as one batch. That makes build_model_constraints safe to call on every build path, including the ones that keep the existing table (a plain incremental run, a DML refresh), so a constraint added to an existing model lands on its next run instead of doing nothing until --full-refresh. What the guard cannot see is a constraint whose definition changes under an unchanged name: a constraint name, unlike a dbt_idx_ index name, is not a hash of its definition, so redefining one still needs --full-refresh. That, and the Msg 3726 a foreign key produces while the referenced model's backup table is dropped, are documented in the README.

The second commit fixes an independent bug found while testing the DML path. It affects models with no constraints at all: a model with table_refresh_method: dml lost its clustered columnstore index the first time its schema changed, and never got it back. That path builds a scratch table with SELECT * INTO and, when the columns no longer match, renames it into position - but SELECT * INTO copies no index and no constraint and takes nullability from the query, and create_indexes only builds what the indexes config names, never the as_columnstore CCI. So the model came back as a heap and stayed one, because every later run matched the new schema and took the DELETE+INSERT path; under an enforced contract the same rename also dropped its NOT NULLs and inline constraints. This looks like it could have been a deliberate trade, but the branch already calls create_indexes, and #641's tests for CCI preservation and for contract enforcement both change only data, never columns, so both stay on the DELETE+INSERT branch, while the one test that does change schema hardcodes as_columnstore: false. The three features were each covered and never crossed.

That branch now rebuilds the scratch through create_table_as before renaming it, which is how every other build path in the adapter creates a table. The rebuild is confined to that branch on purpose: doing it up front would build, and then throw away, a columnstore index on every steady-state refresh, which on a large table dominates the run, and a schema change is rare. It costs a second execution of the model's SQL on that run, the SELECT * INTO probe having already run it once. Probing the tmp view instead of the materialized scratch would avoid that, but it changes how the probe behaves and belongs in its own change.

Adds a functional suite that reads back sys.objects, sys.indexes and sys.masked_columns rather than asserting on generated SQL: a named constraint lands under its declared name and survives a full-refresh rebuild, a primary key on a columnstore table is nonclustered, expression: clustered is honoured, a constraint added to an existing model applies on the next run, an incremental model survives repeated runs, a column-level name: is ignored, a foreign key declared with to: ref(...) resolves, a named constraint coexists with a masked column, and a dml model keeps its columnstore index across a schema change with no contract involved. Adds unit tests for the renderers. Two expectations in the inherited dbt constraint tests encoded the dropped-constraint behaviour as expected SQL and are updated.

Verified locally against SQL Server 2022 (CU26) and SQL Server 2025 (RTM-CU8, 17.0.4075.5), with identical results on both: the unit tests and the full functional suite pass. The behaviour this depends on is the same across the two - a nonclustered primary key coexisting with the clustered columnstore index, constraint names being scoped per schema, and a named constraint applying to a column that carries a data mask.

[Benjamin-Knight]

Can we split the actual bug fix out into its own pull request, that needs to land even constraints is held up in review.

[lll86789]

Done — the DML refresh fix is now #829, sitting on master, so it can go in whenever it's ready.

What's left here is just the constraints work:

• fix(constraints): emit column and model constraints instead of dropping them — unchanged.
• test(constraints): cover the dml refresh's rename-swap keeping constraints — TestDmlRefreshKeepsConstraints, checking that the named PRIMARY KEY, the inline CHECK and the NOT NULLs all survive the rename-swap. It only means anything once the constraint emission in this PR is in, so it stayed; the contract-free columnstore test went over to fix: rebuild the DML refresh's scratch table so a schema change keeps the columnstore #829.

[axellpadilla]

@lll86789 please check conflict

[axellpadilla]

Reviewed and validated locally against SQL Server 2025 (mssql-python backend): unit suite and the full functional suite at -n auto, both green. Pushed three follow-up commits onto this branch before merging:

• Validate a named key's expression before the build. A named primary_key/unique is applied by ALTER after the commit, so a bad expression on one only failed after the new table was swapped in. It now fails when the CREATE TABLE is rendered, like an unnamed one.
• Doc corrections. The nonclustered-index/minimal-logging note was scoped to full_refresh_build: prebuilt, but every contract-enforced model loads as CREATE TABLE + INSERT … WITH (TABLOCK), so the cost applies to all of them. The changelog pointed at table_refresh_method: dml as the foreign-key workaround, which the README (correctly) says is not one; it now points at the drop_fk_constraints() pre_hook. The README also documents the state a Msg 3726 failure leaves behind (new parent in place without its named constraints, child key pointing at __dbt_backup) and how to get out of it.
• Spec + step-by-step e2e tests. docs/constraints.md lists every rule with the test that verifies it; tests/functional/adapter/mssql/test_constraints_e2e.py walks seven scenarios across runs (contract on/off, both violation modes, every "changing a constraint" bullet, the FK parent rebuild and its recovery, clustering override, prebuilt path, several checks on one column, unit tests over a keyed contract).

One observation, not changed here: sqlserver__unit_test_create_table_as has no caller. dbt's unit materialization calls create_table_as(temporary=True, …), which never takes the contract branch, so the only_not_null edit to that macro is inert. The guarantee it describes holds anyway and is covered by the new e2e scenario.

Backport to release/v1.11 follows in a separate PR.