code: function getKeyColumn(propDesc, keyPropContainer) {
  	if (propDesc.keyColumn)
  		return propDesc.keyColumn;
  	const keyPropDesc = keyPropContainer.getPropertyDesc(
  		propDesc.keyPropertyName);
  	return keyPropDesc.column;
  }
docs: Get map key column.
  
  @private
  @param {module:x2node-records~PropertyDescriptor} propDesc Map property
  descriptor.
  @param {module:x2node-records~PropertiesContainer} keyPropContainer Container
  where to look for the key property, if applicable.
  @returns {string} Column name.