implement ALTER table support

Author: tanner0101

Sources/FluentPostgreSQL/PostgreSQLDatabase+SchemaSupporting.swift

@@ -34,7 +34,15 @@ extension PostgreSQLQuery {
                 columns: fluent.createColumns,
                 constraints: fluent.createConstraints
             ))
-        case .alter: fatalError()
+        case .alter:
+            query = .alterTable(.init(
+                ifExists: false,
+                name: fluent.table,
+                addColumns: fluent.createColumns,
+                dropColumns: fluent.deleteColumns.map { $0.name },
+                addConstraints: fluent.createConstraints,
+                dropConstraints: fluent.deleteConstraints.map { $0.name ?? "unknown" }
+            ))
         case .drop:
             query = .dropTable(.init(name: fluent.table, ifExists: false))
         }
@@ -61,7 +69,7 @@ extension PostgreSQLDatabase: SchemaSupporting {
 
     public static func schemaField(for type: Any.Type, isIdentifier: Bool, _ column: PostgreSQLQuery.Column) -> PostgreSQLQuery.ColumnDefinition {
         var constraints: [PostgreSQLQuery.ColumnConstraint] = []
-        let dataType: PostgreSQLQuery.DataType
+        var dataType: PostgreSQLQuery.DataType
 
         var type = type
         if let optional = type as? AnyOptionalType.Type {
@@ -90,10 +98,19 @@ extension PostgreSQLDatabase: SchemaSupporting {
         }
 
         if isIdentifier {
-            switch dataType {
-            case .smallint, .integer, .bigint:
-                constraints.append(.init(.generated(.byDefault)))
-            default: break
+            if _globalEnableIdentityColumns {
+                switch dataType {
+                case .smallint, .integer, .bigint:
+                    constraints.append(.init(.generated(.byDefault)))
+                default: break
+                }
+            } else {
+                switch dataType {
+                case .smallint: dataType = .smallserial
+                case .integer: dataType = .serial
+                case .bigint: dataType = .bigserial
+                default: break
+                }
             }
 
             // create a unique name for the primary key since it will be added

Tests/FluentPostgreSQLTests/FluentPostgreSQLTests.swift

@@ -325,6 +325,7 @@ class FluentPostgreSQLTests: XCTestCase {
             var id: Int?
             let name: String
             let type: PlanetType
+            let test: String?
 
             static func prepare(on conn: PostgreSQLConnection) -> Future<Void> {
                 return PostgreSQLDatabase.create(Planet.self, on: conn) { builder in
@@ -346,6 +347,10 @@ class FluentPostgreSQLTests: XCTestCase {
 
         let rows = try Planet.query(on: conn).filter(\.type == .gasGiant).all().wait()
         XCTAssertEqual(rows.count, 0)
+        
+        try PostgreSQLDatabase.update(Planet.self, on: conn) { builder in
+            builder.field(for: \.test)
+        }.wait()
     }
 
     func testContains() throws {