Fix "NOT NULL column with NULL" error in v5 SQLite migration

Previously, this migration would cause the error: "Cannot add a NOT NULL
column with default value NULL". This fixes that by setting the default
value for new columns to '' (empty string). It updates the query builder
to support this, too.

db/create.go

@@ -177,7 +177,11 @@ func (c *Column) String() (string, error) {
 
 	if c.Default.Set {
 		str.WriteString(" DEFAULT ")
-		str.WriteString(c.Default.Value)
+		val := c.Default.Value
+		if val == "" {
+			val = "''"
+		}
+		str.WriteString(val)
 	}
 
 	if c.PrimaryKey {
@@ -250,4 +254,3 @@ func (b *CreateTableSqlBuilder) ToSQL() (string, error) {
 
 	return str.String(), nil
 }
-

migrations/v5.go

@@ -20,35 +20,35 @@ func oauthSlack(db *datastore) error {
 					Column(
 						"provider",
 						wf_db.ColumnTypeVarChar,
-						wf_db.OptionalInt{Set: true, Value: 24})),
+						wf_db.OptionalInt{Set: true, Value: 24}).SetDefault("")),
 			dialect.
 				AlterTable("oauth_client_states").
 				AddColumn(dialect.
 					Column(
 						"client_id",
 						wf_db.ColumnTypeVarChar,
-						wf_db.OptionalInt{Set: true, Value: 128})),
+						wf_db.OptionalInt{Set: true, Value: 128}).SetDefault("")),
 			dialect.
 				AlterTable("oauth_users").
 				AddColumn(dialect.
 					Column(
 						"provider",
 						wf_db.ColumnTypeVarChar,
-						wf_db.OptionalInt{Set: true, Value: 24})),
+						wf_db.OptionalInt{Set: true, Value: 24}).SetDefault("")),
 			dialect.
 				AlterTable("oauth_users").
 				AddColumn(dialect.
 					Column(
 						"client_id",
 						wf_db.ColumnTypeVarChar,
-						wf_db.OptionalInt{Set: true, Value: 128})),
+						wf_db.OptionalInt{Set: true, Value: 128}).SetDefault("")),
 			dialect.
 				AlterTable("oauth_users").
 				AddColumn(dialect.
 					Column(
 						"access_token",
 						wf_db.ColumnTypeVarChar,
-						wf_db.OptionalInt{Set: true, Value: 512,})),
+						wf_db.OptionalInt{Set: true, Value: 512}).SetDefault("")),
 			dialect.CreateUniqueIndex("oauth_users", "oauth_users", "user_id", "provider", "client_id"),
 		}
 
@@ -63,6 +63,7 @@ func oauthSlack(db *datastore) error {
 							wf_db.ColumnTypeVarChar,
 							wf_db.OptionalInt{Set: true, Value: 128})))
 		}
+
 		for _, builder := range builders {
 			query, err := builder.ToSQL()
 			if err != nil {