DbColumn constructor
not used for now
Implementation
//String? displayName;
DbColumn(String name, {bool allowNull = false, bool unique = false, Object? defaultValue}) {
this.name = name;
this._allowNull = allowNull;
this._unique = unique;
if (defaultValue != null) this._defaultValue = new ValueExpr(defaultValue, this.fieldType);
//if (displayName != null) this.displayName = displayName;
}