My problem: Using Entity Framework with MySQL, I kept getting the error "BLOB/TEXT used in key specification without a key length". I added [StringLength(20)] and [MaxLength(20)] and [Column(TypeName="nvarchar")] to my only string-type [Key], with no progress.
I've found other references to this problem, but they are all out-of-date; there were issues in Connector/Net a few years ago related to this which have since been fixed.
The eventual solution: It was not a [Key] problem at all, despite the error message.
I had used [Index] on some columns which were string-types too, and unlike columns with the [Key] attribute, EF does not automagically set these to an nvarchar type of limited length. Decorate those columns with the attribute [MaxLength(255)], and MySQL is happy.
The OP's solution of setting [MaxLength(255)] attribute does not seem to work with EF Core. I was able to fix the problem by modifying column type from text to varchar(255).
I don't have enough reputation to comment, but the answer given by Ollyver also worked for me with EF6. Adding [MaxLength(255)] fixed the issue. Adding MaxLength also changed the column type from longtext (which was the default) to varchar(255).
