The DecimalType scale defaults to 0

[Myron5787]

When inserting Decimal parameters using Dapper, the digits after the decimal point will be lost.

[DarkWanderer]

Can you provide a minimal reproducible example please?

[Myron5787]

Dapper: 2.1.35 ClickHouse.Client: 7.1.1

var sql = @"INSERT INTO Record
            (
                MemberID, 
                Balance
            )
            VALUES
            (
                @MemberID, //Int64
                @Balance   //Decimal(22,4)
            )";

await using var conn = new ClickHouseConnection($"{connstr}");
await conn.ExecuteAsync(sql, new
{
    MemberID = 1,
    Balance = 1.5M
});

[Myron5787]

Because the ReverseMapping of the decimal value is new Decimal128Type(), and the constructor for Decimal128Type does not set the scale, it defaults to 0.

[DarkWanderer]

This is an interesting one. The issue happens because of the difference between .NET and ClickHouse concepts of decimal. .NET System.Decimal is essentially a floating-point number - there is no specific 'scale' associated with it; whereas ClickHouse decimal always has a specific precision level as part of type information. Dapper cannot infer any information about underlying ClickHouse type - it just passes decimal to DbParameterCollection, which gets translated into a default value

The problem is, whatever default is chosen for scale, there would always be decimal values either overflowing - being too big - or truncating. I.e. it's not strictly speaking possible to deduce ClickHouse type of the target table by just saying 'here's a decimal, figure it out'. Normally, one would solve that problem by using BulkCopy or explicit CH types in query; however, there is no facility for either in Dapper.

The only 'honest' fix I can think right now is to disallow such mapping completely - to avoid giving false expectations. Other ideas are welcome

[DarkWanderer]

Thinking more about it, I think there might be a way to fix it using ClickHouse own data conversion - the corner cases are tricky, but I should have a PR soon

[DarkWanderer]

Please check release 7.2.0

[Myron5787]

After testing, it has been confirmed that the data was written correctly. Thank you.