Drop computed columns before non-computed

[roji]

• Made DropColumnOperation inherit from ColumnOperation. This was needed to distinguish between dropping computed and non-computed, but it makes sense to expose all column facets in case they matter somehow when dropping the column.
• SQL Server and PostgreSQL don't allowing referencing a computed column from another computed column, Sqlite and MySQL do. In any case, we won't parse the computed SQL to understand dependencies.

Fixes #25237

[bricelam]

Instead of storing this in the migrations operation, let's just use the DiffContext instead. The code here would be something like this:

Suggested change

	if (string.IsNullOrWhiteSpace(((ColumnOperation)operation).ComputedColumnSql))
	{
	dropColumnOperations.Add(operation);
	}
	if (string.IsNullOrWhiteSpace(diffContext.FindColumn((DropColumnOperation)operation).ComputedColumnSql)
	{
	dropColumnOperations.Add(operation);
	}

[bricelam]

Also double-check the behavior of null vs whitespace. I think there's an overload of HasComputedColumnSql() that marks the column as computed, but doesn't actually specify the SQL. Although, it probably throws that it's not defined before it gets here...

[roji]

Instead of storing this in the migrations operation, let's just use the DiffContext instead.

Done, check it out (am still worried we'll need facets in SQL generation at some point for dropping stuff, but we can wait for that to happen).

Also double-check the behavior of null vs whitespace. I think there's an overload of HasComputedColumnSql() that marks the column as computed, but doesn't actually specify the SQL. Although, it probably throws that it's not defined before it gets here...

Seems so - I did a quick console test using this PR code, and got the following exception stack trace (BTW should this be model-validated?):

Unhandled exception. System.InvalidOperationException: The computed column SQL has not been specified for the column 'Name.Blogs'. Specify the SQL before using Entity Framework to create the database schema.
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Initialize(ColumnOperation columnOperation, IColumn column, RelationalTypeMapping typeMapping, Boolean isNullable, IEnumerable`1 migrationsAnnotations, Boolean inline) in /home/roji/projects/efcore/src/EFCore.Relational/Migrations/Internal/MigrationsModelDiffer.cs:line 1145
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Add(IColumn target, DiffContext diffContext, Boolean inline)+MoveNext() in /home/roji/projects/efcore/src/EFCore.Relational/Migrations/Internal/MigrationsModelDiffer.cs:line 1090

Test code

await using var ctx = new BlogContext();
await ctx.Database.EnsureDeletedAsync();
await ctx.Database.EnsureCreatedAsync();

public class BlogContext : DbContext
{
    public DbSet<Blog> Blogs { get; set; }

    static ILoggerFactory ContextLoggerFactory
        => LoggerFactory.Create(b => b.AddConsole().AddFilter("", LogLevel.Information));

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        => optionsBuilder
            .UseSqlServer(@"Server=localhost;Database=test;User=SA;Password=Abcd5678;Connect Timeout=60;ConnectRetryCount=0")
            .EnableSensitiveDataLogging()
            .UseLoggerFactory(ContextLoggerFactory);

    protected override void OnModelCreating(ModelBuilder modelBuilder)
        => modelBuilder.Entity<Blog>().Property(b => b.Name).HasComputedColumnSql();
}

public class Blog
{
    public int Id { get; set; }
    public string Name { get; set; }
}

[bricelam]

No, it's a valid model--we just can't create the full schema for it.

[roji]

Hmm, that seems... nuanced... But doesn't really matter.

[ghost]

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[ghost]

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These integrations are possibly never going to report a check, and unblocking auto-merge likely requires a human being to update my configuration to exempt these integrations from requiring a passing check.

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