Magento collection filtering 嵌套where条件的简单写法

只使用$collection->addFieldToFilter()，不使用addAttributeToFilter()或者Zend_Db_Expr（可用于更复杂的where语句）
例如：collection->getSelect()->where(new Zend_Db_Expr("(e.created_at > '2013-01-01 00:00:00' OR e.created_at <'2012-01-01 00:00:00)"));

AND关系
------------------------------------------------------
min_weight < $weight AND
max_weight >= $weight

$_collection->addFieldToFilter('min_weight', array('lt' => $weight))
->addFieldToFilter('max_weight', array('gteq' => $weight));

OR关系
------------------------------------------------------
min_weight > $weight OR
$weight > max_weight

$_collection->addFieldToFilter(
array('min_weight','max_weight'),
array(
array('gt' => $weight),
array('lt' => $weight)
)
);

AND和OR嵌套关系
------------------------------------------------------
min_weight > $weight AND
(
$weight <= max_weight AND max_weight<>NULL
OR
max_weight IS NULL
)

$_collection->addFieldToFilter('min_weight', array('lt' => $weight))
->addFieldToFilter(
array('max_weight','max_weight'),
array(
array('gteq' => $weight, 'notnull' => true),
array('null' => true)
)
);