fixed fetchPage can't set attribute in event

[fanybook]

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• Previous PRs: #PRNumber if this PR is a follow-up

Introduction

when set attribute in event fetched:collection

    this.on('fetched:collection', function(collection, response, options) {
      collection.forEach(model => {
        model.set('abc', 0)
      })
    })

pagination.rowCount is wrong (fetchPage case)

            // ↓↓ the keys is `[ 'count(distinct `test_table`.`id`)', 'abc' ]`
            const keys = Object.keys(result.models[0].attributes);

            // ↓↓ keys.length == 2
            if (keys.length === 1) {
              const key = Object.keys(result.models[0].attributes)[0];
              metadata.rowCount = parseInt(result.models[0].attributes[key]);
            }

Motivation

fix this bug

Proposed solution

            if (keys.length >= 1) {
              const key = keys[0];
              metadata.rowCount = parseInt(result.models[0].attributes[key]);
            }

Current PR Issues

Are there any known issues in this Pull Request? This will help others understand if more work will be needed.

Alternatives considered

Describe any alternative approaches to addressing the same problem that you have thought about, and why you chose this
approach instead.

[ricardograca]

The changes look appropriate but can you fill in the rest of the description template and provide tests for this?

[fanybook]

@ricardograca
I don't know where to write test code. Can you give me a demonstration? I'm a nodejs newbie，i am phper.