Efcore.pg: Add second enum (ForNpgsqlHasEnum) causes error

[macon]

This seems to be a regression of 528.

I'm using:

• Npgsql: 4.0.7
• Npgsql.EntityFrameworkCore.PostgreSQL 2.2.4
• Microsoft.EntityFrameworkCore 2.2.6

As mentioned in 528...

I have an existing Enum (myEnum1) which is deployed to DB.
I add a new Enum (myEnum2).
I run dotnet ef migrations add mySecondMigration
I run dotnet ef migrations script and get error message (below)
I can see the following in the migration script:

migrationBuilder.AlterDatabase()
.Annotation("Npgsql:Enum:myEnum1", "New,Submitted,Failed,AwaitingCapture")
.Annotation("Npgsql:Enum:myEnum2", "New,Submitted,Failed")
.OldAnnotation("Npgsql:Enum:myEnum1", "New,Submitted,Failed,AwaitingCapture");
````

Error message:

System.NotSupportedException: Altering enum type $Npgsql.EntityFrameworkCore.PostgreSQL.Metadata.PostgresEnum isn't supported.
at Npgsql.EntityFrameworkCore.PostgreSQL.Migrations.NpgsqlMigrationsSqlGenerator.GenerateEnumStatements(AlterDatabaseOperation operation, IModel model, MigrationCommandListBuilder builder) in C:\projectsnpgsql-entityframeworkcore-postgresql\src\EFCore.PG\Migrations\NpgsqlMigrationsSqlGenerator.cs:line 704
at Npgsql.EntityFrameworkCore.PostgreSQL.Migrations.NpgsqlMigrationsSqlGenerator.Generate(AlterDatabaseOperation operation, IModel model, MigrationCommandListBuilder builder) in C:\projectsnpgsql-entityframeworkcore-postgresql\src\EFCore.PG\Migrations\NpgsqlMigrationsSqlGenerator.cs:line 646
at Microsoft.EntityFrameworkCore.Migrations.MigrationsSqlGenerator.Generate(MigrationOperation operation, IModel model, MigrationCommandListBuilder builder)
at Microsoft.EntityFrameworkCore.Migrations.MigrationsSqlGenerator.Generate(IReadOnlyList1 operations, IModel model) at Microsoft.EntityFrameworkCore.Migrations.Internal.Migrator.GenerateUpSql(Migration migration) at Microsoft.EntityFrameworkCore.Migrations.Internal.Migrator.GenerateScript(String fromMigration, String toMigration, Boolean idempotent) at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.ScriptMigration(String fromMigration, String toMigration, Boolean idempotent, String contextType) at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_01.b__0()
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
```

[vsvirydau-cl]

Still happening for me with version 3.0.1

[Peter-Optiway]

I'm having this issue in 2.2.4 any chance we could get this backported?

Also it would be nice if you allowed altering enums, adding new values or renaming the entire enum.

[apshoemaker]

This still appears to be an issue in 3.1.0.

[roji]

@apshoemaker, @vsvirydau-cl or someone else, could you please open a new issue with clear instructions on how to reproduce the problem?

Trying to do the following works well:

```c#
class Program
{
static void Main()
{
using var ctx = new BlogContext();
ctx.Database.EnsureDeleted();
ctx.Database.EnsureCreated();
}
}

public class BlogContext : DbContext
{
public DbSet Blogs { get; set; }

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    => optionsBuilder.UseNpgsql("...");

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.HasPostgresEnum<Enum1>();
    // modelBuilder.HasPostgresEnum<Enum2>();  // Uncomment only for second migration
}

}

public enum Enum1
{
Value1,
Value2
};

public enum Enum2
{
Value3,
Value4
};

public class Blog
{
public int Id { get; set; }
public string Name { get; set; }
public List Posts { get; set; }
}

public class Post
{
public int Id { get; set; }
public string Title { get; set; }
}

Steps:

1. Generate first initial migration (one enum in model)
2. Uncomment the line above, adding the second enum to the model
3. Generating the second migration
4. Generate a migration script (`dotnet ef migrations script`).

The result works well and at the end I have both enums in the database:

```sql
CREATE TABLE IF NOT EXISTS "__EFMigrationsHistory" (
    "MigrationId" character varying(150) NOT NULL,
    "ProductVersion" character varying(32) NOT NULL,
    CONSTRAINT "PK___EFMigrationsHistory" PRIMARY KEY ("MigrationId")
);

CREATE TYPE enum1 AS ENUM ('value1', 'value2');

CREATE TABLE "Blogs" (
    "Id" integer NOT NULL GENERATED BY DEFAULT AS IDENTITY,
    "Name" text NULL,
    CONSTRAINT "PK_Blogs" PRIMARY KEY ("Id")
);

CREATE TABLE "Post" (
    "Id" integer NOT NULL GENERATED BY DEFAULT AS IDENTITY,
    "Title" text NULL,
    "BlogId" integer NULL,
    CONSTRAINT "PK_Post" PRIMARY KEY ("Id"),
    CONSTRAINT "FK_Post_Blogs_BlogId" FOREIGN KEY ("BlogId") REFERENCES "Blogs" ("Id") ON DELETE RESTRICT
);

CREATE INDEX "IX_Post_BlogId" ON "Post" ("BlogId");

INSERT INTO "__EFMigrationsHistory" ("MigrationId", "ProductVersion")
VALUES ('20200123161609_initial', '3.1.1');

CREATE TYPE enum2 AS ENUM ('value3', 'value4');

INSERT INTO "__EFMigrationsHistory" ("MigrationId", "ProductVersion")
VALUES ('20200123161621_second', '3.1.1');