[!] add support for `pgx.Batch`, closes #152

[makubit]

This PR introduces functionality to properly test SendBatch() function. In the scope of this PR, there was created new helper structs:BatchElement, Batch, and BatchResults.

An example, how to use batches in tests:

// defining the batch that will be passed to the SendBatch() function
    expectedBatch := mock.NewBatch().
        AddBatchElements(
            NewBatchElement("SELECT *", 1),
            NewBatchElement("SELECT *"),
        )

// using rows implementation to properly simulate Query() and QueryRow() functions in BatchResults interface
    rows := NewRows([]string{"id", "name", "email"}).
        AddRow("some-id-1", "some-name-1", "some-email-1").
        AddRow("some-id-2", "some-name-2", "some-email-2")

// mock for pgx.BatchResults
    batchResultsMock := NewBatchResults().WillReturnRows(rows).AddCommandTag(pgconn.NewCommandTag("SELECT 2"))

// real batch call, that we mocked above
    batch := new(pgx.Batch)
    batch.Queue("SELECT * FROM TABLE", 1)
    batch.Queue("SELECT * FROM TABLE")

// our expectation accepts Batch mock and BatchResults mock (in additional builder function) as parameters
    mock.ExpectSendBatch(expectedBatch).
        WillReturnBatchResults(batchResultsMock)

// the call of the function, here we can use br.AnyFunctionCall() implemented in pgx.BatchResults interface
    br := mock.SendBatch(context.Background(), batch)
    a.NotNil(br)
    r, err := br.Query()

I would appreciate any comments and feedback. ~Magda

[pashagolub]

@makubit would you please check the golint warnings and fix them? Not urgent, just to know where we are with your PR :)

[makubit]

Yes, I will look at the warnings later this week, I was pretty busy recently, so I didn't have much time to look into that.

[pashagolub]

No problem at all! Take your time! I'm not planning to spend much time until the new year anyway :)

[makubit]

@pashagolub PR should be fine now, I resolved the linter issues. Starting this week I have more spare time, so I should be more responsive in case there are some other comments/issues.

[pashagolub]

Also raised jackc/pgx#1878 discussion about making Batch and QueuedQuery properties public

[pashagolub]

Something I cannot implement with the new functionality. Would you please help me to build a test case for the code snippet from the pgx manual:

conn, err := pgx.Connect(ctx, os.Getenv("PGX_TEST_DATABASE"))
if err != nil {
    fmt.Printf("Unable to establish connection: %v", err)
    return
}

batch := &pgx.Batch{}
batch.Queue("select 1 + 1").QueryRow(func(row pgx.Row) error {
    var n int32
    err := row.Scan(&n)
    if err != nil {
        return err
    }

    fmt.Println(n)

    return err
})

batch.Queue("select 1 + 2").QueryRow(func(row pgx.Row) error {
    var n int32
    err := row.Scan(&n)
    if err != nil {
        return err
    }

    fmt.Println(n)

    return err
})

batch.Queue("select 2 + 3").QueryRow(func(row pgx.Row) error {
    var n int32
    err := row.Scan(&n)
    if err != nil {
        return err
    }

    fmt.Println(n)

    return err
})

err = conn.SendBatch(ctx, batch).Close()
if err != nil {
    fmt.Printf("SendBatch error: %v", err)
    return
}

The trick here is every QueuedQuery returns at least one row. But in your code we have only one result set for all queued queries. Am I correct?

[makubit]

Hi, yes, you are correct, I didn't notice that. I will work on some better solutions, so that QueryRow() can have more sophisticated results :)

[pashagolub]

No problem!

I didn't notice that.

Me neither. But before you'll spend more time on this, I have an idea, that this feature should look like ExpectedPrepare. In fact the current implementation of ExpectedPrepare should be deleted, it's just a leftover of how lib/sql works.

So in a few words, we just can replace implementation of ExpectedPrepare with ExpectedBatch plus some renaming and tweaks.

I understand it basically means to throw your code away, sorry about that. I will completely understand if you won't work on this implementation. On the other hand I listed this feature for this year GSoC. Maybe it's worth to work on it during the GSoC program? If you're eligible to be a participant I'd love to accept your proposal. If you're not eligible for participating but eligible to be a mentor, I'd love to onboard you as a GSoC mentor. :)