Sorry, I wasn't clear.
In migrations.cs, for example,
SchemaBuilder.CreateTable("ProductPartRecord", table => table
.Column<string>("Description", column => column.NotNull().WithLength(500) )
would cause _contentManager.Create<ProductPart>("Product") to throw an exception.
I have another place to create a product from fetching an xml feed. This service does not use UpdateEditor, my code is something like,
var p = _contentManager.New<ProductPart>("Product");
p.Description = feed.Description;
AutoroutePart is created properly.
I do have AutomaticAdjustmentOnEdit=true set.
Thanks for you answers.
I settled with Create the object first for simplicity.