Relation passed to #or must be structurally compatible. Incompatible values: [:references]

There is a known issue about it on Github.

According to this comment you might want to override the structurally_incompatible_values_for_or to overcome the issue:

def structurally_incompatible_values_for_or(other)  Relation::SINGLE_VALUE_METHODS.reject { |m| send("#{m}_value") == other.send("#{m}_value") } +    (Relation::MULTI_VALUE_METHODS - [:eager_load, :references, :extending]).reject { |m| send("#{m}_values") == other.send("#{m}_values") } +    (Relation::CLAUSE_METHODS - [:having, :where]).reject { |m| send("#{m}_clause") == other.send("#{m}_clause") }end

Also there is always an option to use SQL:

@items  .joins(:orders)  .where("orders.user_id = ? OR items.available = true", current_user.id)

You can write the query in this good old way to avoid error

@items = @items.joins(:orders).where("items.available = ? OR orders.user_id = ?", true, current_user.id)

Hope that helps!

Hacky workaround: do all your .joins after the .or. This hides the offending .joins from the checker. That is, convert the code in the original question to...

@items =  @items  .where(orders: { user_id: current_user.id})  .or(    @items    .where(available: true)  )  .joins(:orders) # sneaky, but works! 😈

More generally, the following two lines will both fail

A.joins(:b).where(bs: b_query).or(A.where(query))  # error! 😞 

A.where(query).or(A.joins(:b).where(bs: b_query))  # error! 😞 

but rearrange as follows, and you can evade the checker:

A.where(query).or(A.where(bs: b_query)).joins(:b)  # works 😈 

This works because all the checking happens inside the .or() method. It's blissfully unaware of shennanigans on its downstream results.

One downside of course is it doesn't read as nicely.