add unit test for unit round tripping

src/stdatamodels/jwst/datamodels/tests/test_fits.py

@@ -5,7 +5,7 @@
 import pytest
 
 from stdatamodels.jwst import datamodels
-from stdatamodels.jwst.datamodels import ImageModel, JwstDataModel, RampModel
+from stdatamodels.jwst.datamodels import ImageModel, JwstDataModel, RampModel, SpecModel
 
 
 @pytest.fixture
@@ -91,3 +91,18 @@ def test_resave_duplication_bug(tmp_path):
 
     with fits.open(fn1) as ff1, fits.open(fn2) as ff2:
         assert ff1['ASDF'].size == ff2['ASDF'].size
+
+
+def test_units_roundtrip(tmp_path):
+    m = SpecModel()
+    # this next line is required for stdatamodels to cast
+    # spec_table to a FITS_rec (similar to having data assigned
+    # to the attribute)
+    m.spec_table = m.spec_table
+    m.spec_table.columns['WAVELENGTH'].unit = 'nm'
+
+    fn = tmp_path / "test1.fits"
+    m.save(fn)
+
+    m = datamodels.open(fn)
+    assert m.spec_table.columns['WAVELENGTH'].unit == 'nm'