Npgsql.PostgresException: Column cannot be cast automatically to type bytea

Question

Using EF-Core for PostgresSQL, I have an entity with a field of type byte but decided to change it to type byte[]. But when I do migrations, on applying the migration file generated, it threw the following exception:

Npgsql.PostgresException (0x80004005): 42804: column "Logo" cannot be cast automatically to type bytea

I have searched the internet for a solution but all I saw were similar problems with other datatypes and not byte array. Please help.

Horary

Answer 1

The error says exactly what is happening... In some cases PostgreSQL allows for column type changes (e.g. int -> bigint), but in many cases where such a change is non-trivial or potentially destructive, it refuses to do so automatically. In this specific case, this happens because Npgsql maps your CLR byte field as PostgreSQL smallint (a 2-byte field), since PostgreSQL lacks a 1-byte data field. So PostgreSQL refuses to cast from smallint to bytea, which makes sense.

However, you can still do a migration by writing the data conversion yourself, from smallint to bytea. To do so, edit the generated migration, find the ALTER COLUMN ... ALTER TYPE statement and add a USING clause. As the PostgreSQL docs say, this allows you to provide the new value for the column based on the existing column (or even other columns). Specifically for converting an int (or smallint) to a bytea, use the following:

ALTER TABLE tab ALTER COLUMN col TYPE BYTEA USING set_bytea(E'0', 0, col);

If your existing column happens to contain more than a single byte (should not be an issue for you), it should get truncated. Obviously test the data coming out of this carefully.

Forgiving

Thanks for your response. My apologies for my late response. I'm confused as to where to put that line of SQL command in the migration file. This is what I have in the up method of the migration file. migrationBuilder.AlterColumn<byte[]>( name: "Logo", table: "Companies", nullable: true, oldClrType: typeof(byte), oldNullable: true); – Horary

Try replacing that line with a call to migrationBuilder.Sql("..."), placing my proposed ALTER TABLE above in the parameter. For a complete solution you'd have to do the reverse for the down method - casting bytea to smallint - which can be truly destructive as later bytes are dropped. – Forgiving

And of course, test thoroughly on some dev environment before running this on an actual production database. – Forgiving

Thank you very much for your help. Although I had to reset migrations and used a fresh database to continue my work, I made a backup and will try your solution on it. I'm grateful. – Horary

Answer 2

Although the answer given by @Shay Rojansky is correct, I encountered some difficulties when trying to apply it as described above. I had to adjust it a bit to make it work for me:

Let say we have an entity like this:

public class Service
{
    public long Id { get; set; }

    public string? Name { get; set; }

    public string? Image { get; set; }
}

And you decided to change the type of "Image" from "string" to "byte[]" (or from "byte" to "byte[]") and created your migrations after the changes. PostgreSQL will throw an exception stating that "column cannot be automatically converted to type bytea"

As @Shay Rojansky already pointed out, you should go through your migrations file to find something like the following:

migrationBuilder.AlterColumn<byte[]>(
    name: "Image",
    table: "Services",
    type: "bytea",
    nullable: true,
    oldClrType: typeof(string),
    oldType: "text");

If you find it, you need to replace it with:

migrationBuilder.Sql(@"ALTER TABLE ""Services"" ALTER COLUMN ""Image"" TYPE BYTEA USING ""Image""::bytea");

I used the verbatim string literal to escape special characters so that the resulting string is the following in pure SQL:

ALTER TABLE "Services" ALTER COLUMN "Image" TYPE BYTEA USING "Image"::bytea;

For more details, please see these pages:

• https://www.postgresqltutorial.com/postgresql-tutorial/postgresql-change-column-type/
• https://learn.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.migrations.migrationbuilder.altercolumn?view=efcore-8.0
• How to alter a column's data type in a PostgreSQL table?

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