Reverse a nested array in Ruby - ruby-on-rails

Question

I'm attempting to take a JSON API response, with nested associated resources, and reverse the associations in a Rails app.
So, imagine I get a response like this:
{
"spenders": [
{
"id": 1,
"name": "John Doe",
"accounts": [
{
"id": 1,
"name": "Account One"
},
{
"id": 2,
"name": "Account Two"
}
]
},
{
"id": 2,
"name": "Jane Doe",
"accounts": [
{
"id": 1,
"name": "Account One"
},
{
"id": 3,
"name": "Account Three"
}
]
}
]
}
My goal is to convert this into structure like this:
{
"accounts": [
{
"id": 1,
"name": "Account One",
"spenders": [
{
"id": 1,
"name": "Stephen Margheim"
},
{
"id": 2,
"name": "Greg Barendt"
}
]
},
{
"id": 2,
"name": "Account Two",
"spenders": [
{
"id": 1,
"name": "Stephen Margheim"
}
]
},
{
"id": 3,
"name": "Account Three",
"spenders": [
{
"id": 2,
"name": "Greg Barendt"
}
]
}
]
}
Now, I can do this fairly well with iteration over the hash and building a new hash:
spenders_hash = {}
accounts.each do |account|
account.spenders.each do |spender|
if spenders_hash.key? spender.id
spenders_hash[spender.id][:accounts] << account
else
spenders_hash[spender.id] = hash_from_spender_and_account(spender, account)
end
end
end
spenders_hash
def hash_from_spender_and_account(spender, account)
{
id: spender.id,
name: spender.name,
accounts: [account],
}
end
I'm hoping to find [1] a more flexible solution that isn't reliant on knowing the key names in advance and [2] hopefully more efficient.
Thoughts?