HasPrecision() does not override [Column(TypeName=data_type(precision, scale))]

[lajones]

If you set up the code below:

    public class PrecisionContext01 : DbContext
    {
        public DbSet<Precision> Precisions { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer(@"your_connection_string");
        }

        protected override void OnModelCreating(ModelBuilder builder)
        {
            builder.Entity<Precision>()
                .Property(p => p.Score).HasPrecision(14, 2);
        }
    }

    public class Precision
    {
        public int Id { get; set; }
        [Column(TypeName = "decimal(17,5)")]
        public decimal Score { get; set; }
    }

And then do Add-Migration followed by Script-Migration you get the following SQL. I.e. the column type obeyed the annotation and not the fluent API.

CREATE TABLE [Precisions] (
    [Id] int NOT NULL IDENTITY,
    [Score] decimal(17,5) NOT NULL,
    CONSTRAINT [PK_Precisions] PRIMARY KEY ([Id])
);```

[lajones]

We decided the above scenario was by design (column type being an exception to the usual "fluent API trumps annotations" approach). But how about this?

[Column(TypeName = "decimal")] with HasPrecision(14, 2) produces:

CREATE TABLE [Precisions] (
    [Id] int NOT NULL IDENTITY,
    [Score] decimal NOT NULL,
    CONSTRAINT [PK_Precisions] PRIMARY KEY ([Id])
);

i.e. no precision or scale specified in the SQL data type, so SQL Server at least would use its default of decimal(18,2).

.HasColumnType("decimal").HasPrecision(14, 2) without any annotation produces the same thing.

And so does .HasPrecision(14, 2).HasColumnType("decimal") without any annotation.

Both .HasColumnType("decimal(17,5)").HasPrecision(14, 2) and .HasPrecision(14, 2).HasColumnType("decimal(17,5)") without any annotation produce a SQL data type of decimal(17,5).

Summing up: currently HasPrecision() only has any effect if you do not specify the column type either by annotation or by fluent API, regardless of whether the column type includes precision and scale.

Do we want to change any of this?

[lajones]

For comparison, EF6 throws an exception, e.g. "The store type 'decimal(17,5)' could not be found in the SqlServer provider manifest", if the column type contains any precision or scale.

You can set it to just decimal using either [Column(TypeName = "decimal")] or .HasColumnType("decimal"). If a .HasPrecision() call is made as well then the SQL data type respects that. It doesn't matter in which order you specify the fluent API calls.

E.g. .HasPrecision(12, 4).HasColumnType("decimal") results in a SQL data type of decimal(12,4).

[lajones]

Decisions from meeting 6/1/2020:

• Will update the code so that if column type is specified without a precision (or scale) and then the HasPrecision() API is called, then we will respect that i.e. SQL data type should be the combination of the two.
• Will not (for now) issue a warning if user specifies precision/scale using both. We'll keep current functionality (i.e. the column type precision/scale wins) and wait to see if there's any need for the warning.

[lajones]

Found that e.g. HasColumnType("varchar") with .HasMaxLength(nn) didn't work as expected and decisions on this issue were meant to match that - see #21140. So blocking on this til we make a decision on #21140.