On polymorphic associations with has_many through, is it possible to pull in multiple source types with one query?

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Blake LeBlanc

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Mar 19, 2020, 12:25:09 PM3/19/20
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When dealing with polymorphic associations, is it possible to have a has_many through query that pulls in ALL available source_types?

My understanding so far is that each source type needs its own query method, as I show here in the Image model

**image.rb**
```
has_many :image_tags
has_many :tags, through: :image_tags, source: :taggable, source_type: 'Tag'
has_many :people, through: :image_tags, source: :taggable, source_type: 'Person'
has_many :businesses, through: :image_tags, source: :taggable, source_type: 'Business'
...
```

**tag.rb**
```
has_many :image_tags, as: :taggable
has_many :images, through: :image_tags
```

**image_tag.rb**

```
belongs_to :image
belongs_to :taggable, polymorphic: true

def build_taggable(params)
  self.taggable = taggable_type.constantize.new(params)
end
```

What I'd like to be able to do, however, is create one query method that pulls in all associated records regardless of what source_type they might belong to.

Just thinking out loud, would that likely involve creating some sort of raw SQL join that acts directly on the ImageTags table? Or is there a more Railsy/ActiveRecordy way of approaching it? 

Thank you in advance for any help or insights!
-Blake

Ariel Juodziukynas

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Mar 19, 2020, 6:20:33 PM3/19/20
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I'm not sure if you can have all that with a single AR query, I think you need a custom query (I would use a UNION query an I think AR doesn't support that)

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Blake LeBlanc

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Mar 20, 2020, 10:38:45 AM3/20/20
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Thank you Ariel, I'll look into it! Because right now I'm running each source_type query individually then concat-ing the results into a single array. Gets the job done, but it'd be nice to tackle it all in one fell swoop.


On Thursday, March 19, 2020 at 5:20:33 PM UTC-5, Ariel Juodziukynas wrote:
I'm not sure if you can have all that with a single AR query, I think you need a custom query (I would use a UNION query an I think AR doesn't support that)

El jue., 19 mar. 2020 a las 13:25, Blake LeBlanc (<blake....@gmail.com>) escribió:
When dealing with polymorphic associations, is it possible to have a has_many through query that pulls in ALL available source_types?

My understanding so far is that each source type needs its own query method, as I show here in the Image model

**image.rb**
```
has_many :image_tags
has_many :tags, through: :image_tags, source: :taggable, source_type: 'Tag'
has_many :people, through: :image_tags, source: :taggable, source_type: 'Person'
has_many :businesses, through: :image_tags, source: :taggable, source_type: 'Business'
...
```

**tag.rb**
```
has_many :image_tags, as: :taggable
has_many :images, through: :image_tags
```

**image_tag.rb**

```
belongs_to :image
belongs_to :taggable, polymorphic: true

def build_taggable(params)
  self.taggable = taggable_type.constantize.new(params)
end
```

What I'd like to be able to do, however, is create one query method that pulls in all associated records regardless of what source_type they might belong to.

Just thinking out loud, would that likely involve creating some sort of raw SQL join that acts directly on the ImageTags table? Or is there a more Railsy/ActiveRecordy way of approaching it? 

Thank you in advance for any help or insights!
-Blake

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