cheatsheet.cheatmd

# Cheatsheet

> #### Transactions Required {: .info}
>
> References to large objects are only valid for the duration of a
> transaction. In practice, all operations on large objects need to be in
> an `Repo.transaction/1` or `Repo.transact/1` call.
>
> Any large object value will be closed automatically at the end of the
> transaction.

## Operating on objects as a whole

{: .col-2}

### Streaming API

#### Writing local file to an object
```elixir
stream = File.stream!("/tmp/bigfile.dat")
{:ok, object_id} =
  Repo.import_large_object(stream)
```

#### Reading object to local file
```elixir
stream = File.stream!("/tmp/bigfile.dat")
:ok =
  Repo.export_large_object(object_id, into: stream)
```

### Buffered API

#### Writing data to an object
```elixir
large_binary = "This is a large binary."
{:ok, object_id} =
  Repo.import_large_object(large_binary)
```

#### Reading data from an object
```elixir
{:ok, data} =
  Repo.export_large_object(object_id)
```

## Granular access to objects

{: .col-2}

### Create a new object
```elixir
{:ok, object_id} =
  Repo.create_large_object(mode: :write)
```

### Open an existing object
```elixir
# For reading
{:ok, object} =
  Repo.open_large_object(object_id)

# For writing
{:ok, object} =
  Repo.open_large_object(object_id, mode: :write)
```

### Read from object
```elixir
# Read 1024 bytes
{:ok, data} =
  PgLargeObjects.LargeObject.read(object, 1024)
```

### Write to object
```elixir
binary = "Some data to store."
:ok =
  PgLargeObjects.LargeObject.write(object, binary)
```

### Get object size
```elixir
{:ok, size} =
  PgLargeObjects.LargeObject.size(object)
```