问题描述
假设以下数据模式：
Usage
======
client_id
resource
type
amount
Billing
======
client_id
usage_resource
usage_type
rate
在这个例子中，假设我有多个资源，每个资源可以在很多方法。例如，一个资源是小部件。 小部件可以是 foo ed，它们可以是 bar ed 。 gizmo 也可以是 foo ed和 bar ed。这些使用类型以不同的费率收费，对不同的客户来说可能甚至不同。使用情况（资源）的每次出现记录在使用情况表中。每个计费率（用于客户端，资源和类型组合）存储在计费表中。
In this example, suppose I have multiple resources, each of which can be used in many ways. For example, one resource is a widget. Widgets can be fooed and they can be bared. Gizmos can also be fooed and bared. These usage types are billed at different rates, possibly even different rates for different clients. Each occurence of a usage (of a resource) is recorded in the Usage table. Each billing rate (for client, resource, and type combination) is stored in the billing table.
（顺便说一句，如果这个数据模式不是正确的方法来解决这个问题，请提出建议。）
使用Ruby on Rails和ActiveRecord可以创建一个 has_many 从Billings到Usages的关系，以便我可以获得给定计费费用的使用情况列表？有没有一个 has_many，...通过的语法，我不知道？
Is it possible, using Ruby on Rails and ActiveRecord, to create a has_many relationship from Billings to Usages so that I can get a list of usage instances for a given billing rate? Is there a syntax of the has_many, :through that I don't know?
再次，我可以从错误的角度接近这个问题，所以如果你能想到一个更好的方法，请说出来！
Once again, I may be approaching this problem from the wrong angle, so if you can think of a better way, please speak up!
推荐答案
有显然是sourceforge的一个项目来扩展Rails的ActiveRecord，支持。我没有使用这个扩展名，但它可能会帮助你。这也是rubyforge的宝石。
There is apparently an project at sourceforge to extend Rails' ActiveRecord with support for Composite Primary Keys. I haven't used this extension, but it might help you. It's also a gem at rubyforge.
从2.0版开始，纯Ruby on Rails不支持复合主键（参见）。每个表必须有一个名为 id 的单列自动递增键。
Plain Ruby on Rails, as of version 2.0, does not support compound primary keys (cf. HowToUseLegacySchemas). Every table must have a single-column, auto-increment key named "id".
我所看到的解释是：如果要使用旧数据库，您只需要复合主键。这当然是一个荒谬的数据建模视图。
The explanation I have seen is: "You only need compound primary keys if you want to use a legacy database." This is of course a ridiculously ignorant view of data modeling.
我看到的解决方案是：
- Usage.client_id - > Client.id
- Usage.type_id - > Usagetype.id
- Usage.resource_id - >资源.id
- Billing.usage_id - > Usage.id
- Billing.client_id - > Client.id
- Billing.type_id - > Usagetype.id
- Billing.resource_id - > Resource.id
- Usage.client_id -> Client.id
- Usage.type_id -> Usagetype.id
- Usage.resource_id -> Resource.id
- Billing.usage_id -> Usage.id
- Billing.client_id -> Client.id
- Billing.type_id -> Usagetype.id
- Billing.resource_id -> Resource.id
Billing 中明显冗余的外键试图强制执行部分引用完整性。但是它并不完全相同 - 它不会阻止您在 Billing 中创建行，引用中的一行使用使用错误的客户端/资源/ usagetype组合，不符合帐单表中引用行中的那些。
The apparently redundant foreign keys in Billing attempt to enforce partial referential integrity. But it doesn't quite get there -- it does not prevent you from creating rows in Billing that reference a row in Usage with the wrong client/resource/usagetype combination, not matching those in the referencing row in the Billing table.
编辑： @Yarik ： 你是对的。 使用可以引用结算。
edit: @Yarik: yes, you're right. It makes more sense for Usage to reference Billing.
- Usage.billing_id - > Billing.id
嗯。我制作了一个ER图，但我无法将其插入图像。
Hmm. I made an ER diagram but I'm having trouble inserting it as an image.
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