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.
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.