Creating one record that has two associations - ruby-on-rails

Question

I'm trying to create a Transaction that simultaneously is the child of a Request and also one part in a many-to-many relationship of an Inventory.
Model code:
class Transaction < ActiveRecord::Base
belongs_to :request
has_many :transactories
has_many :inventories, through: :transactories
end
class Inventory < ActiveRecord::Base
has_many :transactories
has_many :transactions, through: :transactories
end
class Transactory < ActiveRecord::Base
belongs_to :inventory
belongs_to :transaction
end
Here's the flow I'm trying to achieve:
User POSTs a request that contains additional data in a hash where key = the itemlist_id of what they want and the value = the quantity of that itemlist_id that they want. Let's say user wants two of 9, that would look like this: { 9 => 2}
For each itemlist_id in the user provided hash, I'm going to look inside the Inventories table and pull out the inventory_ids where the itemlist_id matches what the user is looking for and the owner of that inventory_id is not the user him or herself. Let's say that in the Inventories table, there are 3 ids that fulfill this: [X, Y, Z]
Now what I'd like to do is create the Transactions that belong to the Request (Request was already created earlier) and associate the Transactions and Inventories with each other. The outcome of this step is two-fold (I think it's easier to write from the perspective of what the view will look like:
that the owner of each X, Y, and Z inventory_id should see that there are 2 transactions for their item (so they can pick which one they want to honor)
that the user can see that for each of their 2 transactions, there are notifications to the owners of each X, Y, and Z
Code to create the associations
# Assume overarching parent request has been created, called #requestrecord
# Step 1, #transactionparams = { 9 => 2 }
#transactionparams.each do |itemlist_id, quantity|
# Step 2 matched_inventory_id = [X,Y,Z]
matched_inventory_id = Inventory.where.not(signup_id: #requestrecord.signup.id).where(itemlist_id: itemlist_id).ids
# Step 3, 2 transactions created each with itemlist_id of 9, each associated with inventory_ids X, Y, Z. In turn, inventory_ids X, Y, Z each associated with each of the two transactions created
quantity.to_i.times do
transaction = #requestrecord.transactions.create(itemlist_id: itemlist_id)
transaction.inventories.create matched_inventory_id
end
end
The line I can't get right is in step 3:
transaction.inventories.create matched_inventory_id
This throws an error that the parameters for create must be a hash. I also tried:
matched_inventory_id.each do |id|
transaction.inventories.create(inventory_id: id)
end
This failed because inventory_id is not a valid attribute. So... two questions:
How do I associate each of X, Y, Z inventory id with each transaction 1 and 2?
If I write one line of code to achieve above, conceivably (hopefully), I've achieved the reverse association as well? Meaning in a has_many :through, as long as I associate Inventory with Transactions, I'm automatically also associating Transactions with Inventories, right?

Answer 1

Finally someone answered this question here: Creating joined records using has_many :through
Basically I created the Transaction belonging to Request parent first, then associated it with the Inventories like so:
transaction.inventory_ids += matched_inventory_ids
That new line replaced this old line of code:
transaction.inventories.create matched_inventory_id
And yes, once it's associated one way, the two-way works as well since it's a many-to-many relationship.