I want to join records together and separate them with "-"

I know how to join one table records together like this:

```
@keywords = @tweet.hash_tags.join("-")

```

But what if it's HABTM associated tables.

For example.

```
// BRAND MODEL
has_and_belongs_to_many :categories

// CATEGORY MODEL
has_and_belongs_to_many :brands

```

If I do this: 

```
@brands = Brand.all  
@brand_categories = @brands.categories.join("-")

```

I get this result: 

```
#<Category:0x0000010445c928>,#<Category:0x0000010445c7c0>,#<Category:0x0000010445c5e0>,#<Category:0x0000010445c400>,#<Category:0x0000010445c270>

```

Hope you understand my question - thanks.