This is an example of a old EE 2 method in my Publisher add-on and how I converted it to EE 3. I've also included the Behat unit(ish) tests so far for the model. For more information on that see https://github.com/litzinger/ee-behat

ee2-publisher_phrase_model.php

<?php
/*
Old EE 2 version of a method on a "model" to get all translations for a phrase.
Its one method that does it all. There are no model properties, collections, or
phrase entities. Its just a query and dumb array as a result.
*/

public function get_translations($phrase_id = null, $status = PUBLISHER_STATUS_OPEN, $lang_id = null, $site_id = null)
{
    if ( !$phrase_id)
    {
        show_error('$phrase_id is required. publisher_phrase.php->get_translations()');
    }

    $site_id = $site_id ?: ee()->publisher_lib->site_id;

    $qry = ee()->db->select('upd.*, up.*, up.id as phrase_id, upd.id as row_id')
        ->from('publisher_phrases AS up')
        ->join('publisher_phrase_data AS upd', 'upd.phrase_id = up.id', 'left')
        ->where('up.id', $phrase_id)
        ->where('upd.publisher_status', $status)
        ->where('up.site_id', $site_id)
        ->get();

    $phrases = array();
    $translations = array();

    foreach ($qry->result() as $row)
    {
        $phrases[$row->publisher_lang_id] = $row;
    }

    if ($lang_id)
    {
        $translations[$lang_id] = isset($phrases[$lang_id]) ? $phrases[$lang_id] : $phrases[ee()->publisher_lib->default_lang_id];
    }
    else
    {
        $languages = ee()->publisher_language->get_enabled_languages();

        foreach ($languages as $lang_id => $language)
        {
            // If we have existing phrase data
            if (isset($phrases[$lang_id]))
            {
                $translations[$lang_id] = $phrases[$lang_id];
            }
            // For some reason we don't have a corresponding phrase_data row
            // create the vars with an empty translation value so the view doesn't bomb.
            else
            {
                $phrases[$lang_id] = new stdClass();
                $phrases[$lang_id]->phrase_id = $phrase_id;
                $phrases[$lang_id]->publisher_lang_id = $lang_id;
                $phrases[$lang_id]->phrase_value = '';

                $translations[$lang_id] = $phrases[$lang_id];
            }

            $translations[$lang_id]->text_direction = ee()->publisher_language->get_language($lang_id, 'direction');
        }
    }

    return $translations;
}

ee3-Phrase.php

<?php
/*
This may look like more code because it is. Its the entire model with relationships defined
with properties and getters/setters. The getTranslations() method works with models
and collections instead of a dumb array.
 */ 
namespace Publisher\Model;

use EllisLab\ExpressionEngine\Service\Model\Collection;
use EllisLab\ExpressionEngine\Service\Model\Query\Builder;
use Publisher\Service\Request;

/**
 * @package     ExpressionEngine
 * @subpackage  Models
 * @category    Publisher
 * @author      Brian Litzinger
 * @copyright   Copyright (c) 2012, 2013 - Brian Litzinger
 * @link        http://boldminded.com/add-ons/publisher
 * @license
 *
 * Copyright (c) 2015. BoldMinded, LLC
 * All rights reserved.
 *
 * This source is commercial software. Use of this software requires a
 * site license for each domain it is used on. Use of this software or any
 * of its source code without express written permission in the form of
 * a purchased commercial or other license is prohibited.
 *
 * THIS CODE AND INFORMATION ARE PROVIDED "AS IS" WITHOUT WARRANTY OF ANY
 * KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A
 * PARTICULAR PURPOSE.
 *
 * As part of the license agreement for this software, all modifications
 * to this source must be submitted to the original author for review and
 * possible inclusion in future releases. No compensation will be provided
 * for patches, although where possible we will attribute each contribution
 * in file revision notes. Submitting such modifications constitutes
 * assignment of copyright to the original author (Brian Litzinger and
 * BoldMinded, LLC) for such modifications. If you do not wish to assign
 * copyright to the original author, your license to  use and modify this
 * source is null and void. Use of this software constitutes your agreement
 * to this clause.
 */

class Phrase extends AbstractModel
{
    const NAME = 'publisher:Phrase';

    protected static $_primary_key = 'id';
    protected static $_table_name = 'publisher_phrases';

    protected $id;
    protected $site_id;
    protected $group_id;
    protected $phrase_name;

    protected static $_relationships = array(
        'ee:Site' => array(
            'type' => 'BelongsTo',
        ),
        'PhraseGroup' => array(
            'type' => 'BelongsTo',
        ),
        'Translations' => array(
            'type'   => 'HasMany',
            'model'  => 'PhraseTranslation',
            'weak'	 => true,
            'to_key' => 'phrase_id',
        ),
    );

    /**
     * @param $phraseName
     * @return Phrase
     */
    public function findPhraseByName($phraseName)
    {
        /** @var Builder $query */
        $query = $this->getQueryBuilder(self::NAME);

        $query
            ->filter('phrase_name', $phraseName);
            
        // I could/should emit an event here and use a subscriber
        // to attach the translations to the phrase intead of loading
        // the Language model and Request service in the getTranslations() method.

        return $query->first();
    }

    /**
     * @param $phraseId
     * @return Phrase
     */
    public function findPhraseById($phraseId)
    {
        /** @var Builder $query */
        $query = $this->getQueryBuilder(self::NAME);

        $query
            ->filter('id', $phraseId);

        return $query->first();
    }

    /**
     * @return string
     */
    public function getTableName()
    {
        return self::$_table_name;
    }

    public function createTable()
    {
        if (ee()->db->table_exists($this->getTableName())) {
            return;
        }

        ee()->dbforge->add_field(array(
            'id'                 => array('type' => 'int', 'constraint' => 10, 'unsigned' => true, 'auto_increment' => true),
            'site_id'            => array('type' => 'int', 'constraint' => 4, 'unsigned' => true, 'default' => 1),
            'group_id'           => array('type' => 'int', 'constraint' => 4, 'unsigned' => true),
            'phrase_name'        => array('type' => 'text'),
            'phrase_desc'        => array('type' => 'text'),
        ));

        ee()->dbforge->add_key('id', true);
        ee()->dbforge->add_key('site_id');

        ee()->dbforge->create_table($this->getTableName());
    }

    /**
     * @return mixed
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * @param mixed $id
     */
    public function setId($id)
    {
        $this->setRawProperty('id', $id);

        return $this;
    }

    /**
     * @return mixed
     */
    public function getSiteId()
    {
        return $this->site_id;
    }

    /**
     * @param mixed $site_id
     */
    public function setSiteId($site_id)
    {
        $this->setRawProperty('site_id', $site_id);

        return $this;
    }

    /**
     * @return mixed
     */
    public function getGroupId()
    {
        return $this->group_id;
    }

    /**
     * @param mixed $group_id
     */
    public function setGroupId($group_id)
    {
        $this->setRawProperty('group_id', $group_id);

        return $this;
    }

    /**
     * @return mixed
     */
    public function getPhraseName()
    {
        return $this->phrase_name;
    }

    /**
     * @param mixed $phrase_name
     */
    public function setPhraseName($phrase_name)
    {
        $this->setRawProperty('phrase_name', $phrase_name);

        return $this;
    }

    /**
     * @return PhraseGroup
     */
    public function getGroup()
    {
        return $this->PhraseGroup;
    }

    /**
     * @return PhraseTranslation[]
     */
    public function getTranslations($status = null)
    {
        /** @var Language $languageModel */
        $languageModel = ee('Model')->make(Language::NAME);
        $languages = $languageModel->findAllEnabledLanguages();
        $defaultLanguage = $languageModel->findDefaultLanguage();

        if (!$status) {
            /** @var Request $request */
            $request = ee(Request::NAME);
            $status = $request->getCurrentStatus();
        }

        /** @var Collection $translations */
        $translationsCollection = $this->Translations
            ->filter('status', $status);

        /** @var Collection $defaultTranslationCollection */
        $defaultTranslationCollection = $this->Translations
            ->filter('lang_id', $defaultLanguage->getId())
            ->filter('status', $status);

        /** @var PhraseTranslation $defaultTranslation */
        $defaultTranslation = $defaultTranslationCollection->first();

        // Iterate all enabled languages. If there is no translation for the language,
        // then set the translation to the default language so the translated phrase
        // collection is the same in all languages.
        foreach ($languages as $language) {
            /** @var Collection $matchingTranslation */
            $matchingTranslationCollection = $this->Translations
                ->filter('lang_id', $language->getId())
                ->filter('status', $status);

            if ($matchingTranslationCollection->count() == 0) {
                /** @var PhraseTranslation $appendedTranslation */
                $appendedTranslation = $defaultTranslation;
                $appendedTranslation->setPhraseValue($defaultTranslation->getPhraseValue());
                $translationsCollection->add($appendedTranslation);
            }
        }

        return $translationsCollection;
    }
}

ee3-Usage.php

<?php
/*
The best part about defining the relationships on the model is you get
access to all the relationships. $phrase->getGroup() or $phrase->PhraseGroup
returns the related group model. Same for $phrase->Translations. If I
didn't have extra filtering to perform on the translations the model
would be much smaller, yet I still get all the data needed.
*/

/** @var Phrase $phraseModel */
$phraseModel = ee('Model')->make(Phrase::NAME);
$phrase = $phraseModel->findPhraseById(1);

/** @var PhraseTranslation[] $translations */
$translations = $phrase->getTranslations();

PhraseContext.php

<?php
use Behat\Behat\Context\Context;
use Behat\Gherkin\Node\TableNode;
use Publisher\Model\Phrase;
use Publisher\Model\PhraseGroup;
use Publisher\Model\PhraseTranslation;
use Publisher\Service\Request;

class PhraseContext implements Context
{
    /**
     * @Given /^I have phrases:$/
     */
    public function iHavePhrases(TableNode $table)
    {
        /** @var Phrase $phraseModel */
        $phraseModel = ee('Model')->make(Phrase::NAME);
        $phraseModel->createTable();

        $rows = $table->getColumnsHash();

        foreach ($rows as $row) {
            ee()->db->insert($phraseModel->getTableName(), $row);
        }
    }

    /**
     * @Given /^I have phrase translations:$/
     */
    public function iHavePhraseTranslations(TableNode $table)
    {
        /** @var PhraseTranslation $phraseTranslationModel */
        $phraseTranslationModel = ee('Model')->make(PhraseTranslation::NAME);
        $phraseTranslationModel->createTable();

        $rows = $table->getColumnsHash();

        foreach ($rows as $row) {
            ee()->db->insert($phraseTranslationModel->getTableName(), $row);
        }
    }

    /**
     * @Given /^I have phrase groups:$/
     */
    public function iHavePhraseGroups()
    {
        /** @var PhraseGroup $phraseGroupModel */
        $phraseGroupModel = ee('Model')->make(PhraseGroup::NAME);
        $phraseGroupModel->createTable();
    }

    /**
     * @Then /^I have the phrase "([^"]*)" with the id (\d+)$/
     */
    public function iHaveThePhrase($phraseName, $expectedId)
    {
        $phrase = $this->findPhraseByName($phraseName);

        PHPUnit_Framework_Assert::assertEquals($expectedId, $phrase->getId());
    }

    /**
     * @Given /^"([^"]*)" has (\d+) translations$/
     */
    public function hasTranslations($phraseName, $expectedCount)
    {
        $this->setStatus('open');
        $phrase = $this->findPhraseByName($phraseName);

        /** @var PhraseTranslation[] $translations */
        $translations = $phrase->getTranslations();

        PHPUnit_Framework_Assert::assertEquals($expectedCount, count($translations));
    }

    /**
     * @Given /^"([^"]*)" translations equal "([^"]*)"$/
     */
    public function translationsEqual($phraseName, $expectedTranslations)
    {
        $this->setStatus('open');
        $phrase = $this->findPhraseByName($phraseName);

        /** @var PhraseTranslation[] $translations */
        $translations = $phrase->getTranslations();

        $translationArray = [];
        foreach ($translations as $index => $translation) {
            $translationArray[] = $translation->getPhraseValue();
        }

        PHPUnit_Framework_Assert::assertEquals($expectedTranslations, $translationArray);
    }

    private function setStatus($status)
    {
        /** @var Request $request */
        $request = ee(Request::NAME);
        $request->setCurrentStatus($status);
    }

    private function findPhraseByName($phraseName)
    {
        /** @var Phrase $phraseModel */
        $phraseModel = ee('Model')->make(Phrase::NAME);
        return $phraseModel->findPhraseByName($phraseName);
    }
    
    /**
     * @Transform /^\[(.*)\]$/
     */
    public function castStringToArray($string)
    {
        return explode(',', $string);
    }
}

phrases.feature.yaml

  Background:
    Given I have phrases:
      | id | site_id | group_id | phrase_name    | phrase_desc |
      | 1  | 1       | 2        | language_en    |             |
      | 2  | 1       | 2        | language_de    |             |
      | 3  | 1       | 2        | language_fr    |             |
      | 4  | 1       | 2        | language_it    |             |
      | 5  | 1       | 1        | some-phrase    |             |
      | 6  | 1       | 1        | another_phrase |             |
      | 7  | 1       | 1        | third_phrase   |             |

    And I have phrase translations:
      | id | phrase_id | site_id | lang_id | status | edit_date  | edit_by | phrase_value   |
      | 1  | 1         | 1       | 1       | open   | 1438629647 | 1       | English        |
      | 2  | 2         | 1       | 1       | open   | 1426683210 | 1       | German         |
      | 27 | 2         | 1       | 2       | open   | 1426683210 | 1       | Deutsche       |
      | 28 | 2         | 1       | 2       | draft  | 1426683210 | 1       | Deutsche DRAFT |
      | 3  | 3         | 1       | 1       | open   | 1426871335 | 1       | French         |
      | 4  | 3         | 1       | 2       | open   | 1426871335 | 1       |                |
      | 5  | 4         | 1       | 1       | open   | 1430436699 | 1       | Italian        |
      | 6  | 4         | 1       | 3       | open   | 1430436699 | 1       |                |
      | 7  | 4         | 1       | 2       | open   | 1430436699 | 1       |                |
      | 9  | 1         | 1       | 3       | open   | 1438629647 | 1       | Anglais        |
      | 11 | 1         | 1       | 2       | open   | 1438629647 | 1       | Englisch       |
      | 13 | 1         | 1       | 4       | open   | 1438629647 | 1       | Inglese        |
      | 15 | 5         | 1       | 1       | open   | 1439559354 | 1       | value          |
      | 16 | 5         | 1       | 3       | open   | 1439559354 | 1       | vfr            |
      | 17 | 5         | 1       | 2       | open   | 1439559354 | 1       | vde            |
      | 18 | 5         | 1       | 4       | open   | 1439559354 | 1       | vit            |
      | 19 | 6         | 1       | 1       | open   | 1446816966 | 1       | a              |
      | 20 | 6         | 1       | 3       | open   | 1446816966 | 1       | b              |
      | 21 | 6         | 1       | 2       | open   | 1446816966 | 1       | c              |
      | 22 | 6         | 1       | 4       | open   | 1446816966 | 1       |                |
      | 23 | 7         | 1       | 1       | open   | 1439555242 | 1       |                |
      | 24 | 7         | 1       | 3       | open   | 1439555242 | 1       |                |
      | 25 | 7         | 1       | 2       | open   | 1439555242 | 1       |                |
      | 26 | 7         | 1       | 4       | open   | 1439555242 | 1       |                |

    And I have phrase groups:

  Scenario: Test phrases
    Then I have the phrase "language_en" with the id 1
    And "language_en" has 4 translations
    And "language_en" translations equal "[English,Anglais,Englisch,Inglese]"
    Then I have the phrase "language_de" with the id 2
    And "language_de" has 4 translations
    And "language_de" translations equal "[German,Deutsche,German,German]"