feat: Add unique rule to dy.Column

[gab23r]

Motivation

Closes #313

Changes

Add the new rule using the same logic than primary_keys.

Drive by:

• Allow primary_keys for array dtype as it now works in polars, I added a test of it.
• Disallow primary_keys for object dtype as it never worked. (technically breaking but IMHO shouldn't break anything)

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a per-column unique constraint to dy.Column/dy.Schema, aligning validation and SQLAlchemy output, and updates array/object primary key behavior.

Changes:

• Introduce unique as a first-class column attribute and emit unique=True in SQLAlchemy column definitions.
• Add schema validation rules for unique columns (and tighten primary key validation via is_unique()).
• Expand tests for unique constraints and enable primary keys on Array columns while disallowing them for Object columns.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/schema/test_validate.py	Adds validation tests for unique columns and Schema.unique_columns().
tests/schema/test_sample.py	Adds sampling tests ensuring generated data respects unique constraints.
tests/column_types/test_array.py	Adds coverage for primary keys on Array columns.
dataframely/columns/_base.py	Adds unique attribute to Column and passes it to SQLAlchemy columns.
dataframely/_base_schema.py	Adds unique rules into schema validation; uses is_unique() for primary keys.
dataframely/columns/array.py	Allows primary_key on arrays and threads through unique.
dataframely/columns/string.py	Threads unique through to Column.
dataframely/columns/integer.py	Threads unique through to Column.
dataframely/columns/float.py	Threads unique through to Column.
dataframely/columns/decimal.py	Threads unique through to Column.
dataframely/columns/datetime.py	Threads unique through to Column for date/time/datetime/timedelta.
dataframely/columns/enum.py	Threads unique through to Column.
dataframely/columns/categorical.py	Threads unique through to Column.
dataframely/columns/list.py	Threads unique through to Column.
dataframely/columns/struct.py	Threads unique through to Column.
dataframely/columns/object.py	Removes primary_key kwarg from Object column constructor.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7c73bb1) to head (04b97f3).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

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##              main      #325   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           56        56           
  Lines         3399      3408    +9     
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+ Hits          3399      3408    +9     

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[Andreas Albert (AndreasAlbertQC)]

Thanks gab23r ! This looks quite nice to me already. Small suggestions.

[Andreas Albert (AndreasAlbertQC)]

Thanks gab23r !

[Oliver Borchert (borchero)]

Meta-comment because I just noticed: pl.struct(primary_key).is_unique() is likely much more inefficient than pl.col(primary_key).is_unique() if we only have a single primary key. We might want to introduce an optimization for this after benchmarking 😄

[Oliver Borchert (borchero)]

Sorry I didn't review previously but I find this implementation suboptimal. Why is it on the schema if we do not check composite uniqueness but uniqueness of individual columns? This should be on the column which would also allow for much more efficient evaluation of is_unique for primitive types (because we can very easily skip the struct-wrapping).

[Oliver Borchert (borchero)]

Besides, this also breaks for nested types; for example setting unique on a list element is simply ignored.