Add ColumnTypeMap which allows bespoke customisation of column types.

[RParkerHp]

• [Y] Do only one thing
• [Y] Non breaking API changes
• [Y] Tested

What did this pull request do?

• Adds WithColumnTypeMap which mirrors functionality of WithDataTypeMap but for columns.

User Case Description

• For varchar, nvarchar, etc, I need to get the length of the field (e.g. nvarchar(25).

  This is implemented in the following way:

var customColumnMap = map[string]func(columnType gorm.ColumnType) (dataType string){
   "varchar":  getColumnTypeAndSize,
}

cfg.WithColumnTypeMap(customColumnMap)

func getColumnTypeAndSize(columnType gorm.ColumnType) (dataType string) {
	typ, ok := columnType.ColumnType()
	if ok {
		len, ok := columnType.Length()
		if ok && len > 0 {
			typ = fmt.Sprintf("%v(%v)", typ, len)
		} else if len == -1 && strings.Contains(typ, "char") {
			typ += "(max)"
		}

		return typ
	}

	panic(fmt.Errorf("unable to process column '%v'", columnType.Name()))
}

[dushaoshuai]

Adds helper.SetDataType in order to override and add data type values.

The value used for bit in the dataType map does not match my requirement that it contains the value bool (rather than []uint8), hence the creation of SetDataType function, which allows this value to be modified. This also allows a new mapping to be created: 'uniqueidentifier' to 'uuid.UUID'.

Is it WithDataTypeMap, or am I missing something?

[RParkerHp]

Adds helper.SetDataType in order to override and add data type values.
The value used for bit in the dataType map does not match my requirement that it contains the value bool (rather than []uint8), hence the creation of SetDataType function, which allows this value to be modified. This also allows a new mapping to be created: 'uniqueidentifier' to 'uuid.UUID'.

Is it WithDataTypeMap, or am I missing something?

You are absolutely right. Sorry about that. I have reverted the GetDataType and SetDataType changes.