Plan --select-model doesn't allow adding unmodified models to an existing dev environment #5497

plan --select-model A dev creates a dev environment, even if there are no changes to the selected model compared to prod. However, it is not possible to add an unmodified model B to the existing environment dev with plan --select-model B dev. I'm unsure whether this is intended.

How to reproduce

Execute the following commands (demo duckdb project):

1. sqlmesh init
2. sqlmesh plan
3. sqlmesh plan --select-model sqlmesh_example.incremental_model dev
4. sqlmesh plan --select-model sqlmesh_example.incremental_model --select-model sqlmesh_example.full_model dev

Current behavior

Step 4. results in

No changes to plan: project files match the dev environment

Expected behavior

Model sqlmesh_example.full_model gets added to the dev virtual environment.

Notes

1. Adding unmodified models to an existing virtual environment can be useful, I think (e.g. when working with pinned_environments or for testing purposes).
2. If step 3. is omitted, the dev environment will be created with both models sqlmesh_example.incremental_model and sqlmesh_example.full_model. So, one possible workaround is to create a new virtual environment instead of updating the old one.

Sqlmesh version

0.224.0