Ecto.Multi BankAccount Example

ecto_multi_bank_account_example.ex

deposit_query = from(BankAccount, where: [id: 2]) #deposit target bank account 
withdraw_query = from(BankAccount, where: [id: 1]) #withdraw from bank account 

deposit_update = [inc: [balance: 100]]
withdraw_update = [inc: [balance: -100]


alias Ecto.Multi
# iex> h Ecto.Multi # Multi.new() creates a one-time op that atomically groups transactions
# iex> h Ecto.Multi.update_all # def update_all(multi, name, queryable_or_fun, updates, opts \\ [])


Multi.new()
|> Multi.update_all(:deposit, deposit_query, deposit_update) 
|> Multi.update_all(:withdraw, withdraw_query, withdraw_update)

##output
%Ecto.Multi{
  names: #MapSet<[:deposit, :withdraw]>,
  operations: [
    withdraw: {:update_all, 
      #Ecto.Query<from a0 in MyApp.BankAccount, where: a0.id == 1>,
      [inc: [balance: -100]], []},
    deposit: {:update_all, 
      #Ecto.Query<from a0 in MyApp.BankAccount, where: a0.id == 2>,
      [inc: [balance: 100]], []},}
    ]
 }

#no logging yet because we havent ran the transaction yet

Multi.new()
|> Multi.update_all(:deposit, deposit_query, deposit_update) 
|> Multi.update_all(:withdraw, withdraw_query, withdraw_update)
|> MyApp.Repo.transaction


##output
# [debug] Query OK db=0.2ms
# begin []
# [debug] QUERY OK source="bank_accounts" db=0.8ms
# UPDATE "bank_accounts" AS a0 SET "balance" = a0."balance" + $1 WHERE (a0."id" = 2) [50]
# [debug] QUERY OK source="bank_accounts" db=0.3ms
# UPDATE "bank_accounts" AS a0 SET "balance" = a0."balance" + $1 WHERE (a0."id" = 1) [-50]
# [debug] Query OK db=0.7ms
# commit []
# {:ok, %{deposit: {1, nil}, withdraw: {1, nil}}}

# https://hexdocs.pm/ecto/composable-transactions-with-multi.html