a commercial culture in which happiness is not just an ideal but an ideology.

【划线部分的翻译】在这种文化中，快乐不仅是一种理想，而且是一种意识形态。

划线部分是定语从句。which 是关系代词，指代 culture，介词 in 提前。所以，in which 意思是：在这种文化中。

知识拓展：“介词 + 关系代词” 结构 

“介词+which”就是把介词提到关系代词 which 之前，它是定语从句中常见的语言现象，也是网友们不易掌握的一个语法项目，问题的关键是这个“介词”较难确定。但它是有规律可循的，有以下几种情况：

1. “介词 + which”可以替代关系副词 when, where, why

(1)替代when

I'll never forget the day on which (=when) my son was born.

我永远忘不了我儿子出生的那一天。（on依据on the day确定）

The year in which (=when) the earthquake happened was 2008.

发生地震的那年是2008年。（in依据in the year确定）

(2) 替代where

This is the office in which (=where) I used to work.这就是我过去工作过的办公室。（in依据in the office确定）

He forgot the exact place in which (=where) he had hidden the jewelry.

他忘记了埋藏珠宝的确切地点。（in依据in the place确定）

(3) 替代why

I'd like you to explain the reason for which (=why) you refused my offer.

我想让你解释一下你拒绝我的帮助的原因。（for依据for the reason确定）

2. 依据与先行词搭配的具体意义确定使用什么介词

I'll never forget the time during which I was with my friends in the country.

我永远不会忘记和朋友们一起在乡村度过的时光。(during依据during the time确定)

I will offer you a better way in which you can do the job.

我要给你提供一个做这项工作较好的办法。(in依据in a better way确定)

3. 依据定语从句中动词或形容词的搭配来确定介词

She bought several clothes, on which she spent all her money.

她买了几件衣服，这些衣服花光了她所有的钱。（on依据spend…on确定）

It is the grammar rules of which I am not sure.

我没有把握的就是语法规则。（of依据be sure of确定）

4. 根据定语从句所表达的具体意思来确定介词

Can you see the river across which there is a bridge?

你能看见上面有座桥的那条河？

He is swimming in the river, on which there is a boat.

他在那条河里游泳，河面上有一只小船。

Here is the river in which I used to swim.

这儿就是我过去常在里面游泳的那条河。

（请网友认真阅读体会，就不要再追问了，好吧？）