EF Core Code-first extension to migrate C# enums from ints in Postgres to PG enums

MigrationsExtensions.cs

using Microsoft.EntityFrameworkCore.Migrations;
using Npgsql.NameTranslation;

namespace Celestial.Database.Extensions;

public static class MigrationsExtensions
{
	/// <summary>
	/// Convert int column to PG enum column using c# enum which was previously mapped to this column
	/// </summary>
	public static void AlterIntToEnum<TEnum>(this MigrationBuilder migrationBuilder, string tableName, string columnName)
		where TEnum : Enum
	{
		// Get TEnum name as postgres enum name (e.g. from "UserType" to "user_type")
		var pgEnumType = NpgsqlSnakeCaseNameTranslator.ConvertToSnakeCase(typeof(TEnum).Name);

		var tempColumnName = $"{columnName}_temp";

		// Get list with all enum values
		var allEnumValues = Enum.GetValues(typeof(TEnum)).Cast<TEnum>().ToList();
		var defaultValue = allEnumValues.First();

		// Add temp column to store PG enum values
		migrationBuilder.AddColumn<TEnum>(
			name: tempColumnName,
			table: tableName,
			type: pgEnumType,
			nullable: false,
			defaultValue: defaultValue);

		var defaultPgValue = NpgsqlSnakeCaseNameTranslator.ConvertToSnakeCase(defaultValue.ToString());

		// Write sql code for "CASE" statement to convert int to PG enums
		var sqlCaseCode = string.Join(" ", allEnumValues.Select(x =>
			$"WHEN {Convert.ToInt32(x)} THEN '{NpgsqlSnakeCaseNameTranslator.ConvertToSnakeCase(x.ToString())}'"));

		// Update temporary columns with enum values
		migrationBuilder.Sql($"UPDATE \"{tableName}\" " +
		                     $"SET \"{tempColumnName}\" = CASE \"{columnName}\" " +
		                     $"{sqlCaseCode} ELSE '{defaultPgValue}' " +
		                     $"END::{pgEnumType};");

		// Drop original column with int values
		migrationBuilder.DropColumn(
			name: columnName,
			table: tableName);

		// Rename temp column to original column name
		migrationBuilder.RenameColumn(
			name: tempColumnName,
			table: tableName,
			newName: columnName);
	}
}

For schema specific cases:

public static void AlterIntToEnum<TEnum>(this MigrationBuilder migrationBuilder, string tableName, string columnName, bool nullable = false, string? schema = null)
        where TEnum : Enum
    {
        // Get TEnum name as postgres enum name (e.g. from "UserType" to "user_type")
        var pgEnumType = NpgsqlSnakeCaseNameTranslator.ConvertToSnakeCase(typeof(TEnum).Name);

        if (schema is not null)
        {
            pgEnumType = schema + "." + pgEnumType;
        }

        var tempColumnName = $"{columnName}_temp";

        // Get list with all enum values
        var allEnumValues = Enum.GetValues(typeof(TEnum)).Cast<TEnum>().ToList();
        var defaultValue = NpgsqlSnakeCaseNameTranslator.ConvertToSnakeCase(
            allEnumValues.First().ToString()
            );

        // Add temp column to store PG enum values
        migrationBuilder.AddColumn<TEnum>(
            name: tempColumnName,
            table: tableName,
            type: pgEnumType,
            nullable: nullable,
            schema: schema,
            defaultValue: defaultValue);

        var defaultPgValue = NpgsqlSnakeCaseNameTranslator.ConvertToSnakeCase(defaultValue.ToString());

        // Write sql code for "CASE" statement to convert int to PG enums
        var sqlCaseCode = string.Join(" ", allEnumValues.Select(x =>
            $"WHEN {Convert.ToInt32(x)} THEN '{NpgsqlSnakeCaseNameTranslator.ConvertToSnakeCase(x.ToString())}'"));

        var qualifiedTableName = schema is null ? $"\"{tableName}\"" : $"{schema}.\"{tableName}\"";

        // Update temporary columns with enum values
        migrationBuilder.Sql($"UPDATE {qualifiedTableName} " +
                             $"SET \"{tempColumnName}\" = CASE \"{columnName}\" " +
                             $"{sqlCaseCode} ELSE '{defaultPgValue}' " +
                             $"END::{pgEnumType};");

        // Drop original column with int values
        migrationBuilder.DropColumn(
            name: columnName,
            schema: schema,
            table: tableName);

        // Rename temp column to original column name
        migrationBuilder.RenameColumn(
            name: tempColumnName,
            table: tableName,
            schema: schema,
            newName: columnName);
    }