SilverStripe\ORM\DataObjectSchema

Complexity

Total Complexity

118

Size/Duplication

Total Lines	927
Duplicated Lines	0 %

Coupling/Cohesion

Components	1
Dependencies	7

Importance

Changes

0

31 Methods

Rating	Name	Duplication	Size	Complexity
A	reset()	0	5	1
A	getTableNames()	0	4	1
A	sqlColumnForField()	0	7	2
A	tableName()	0	8	2
A	baseDataClass()	0	11	3
A	baseDataTable()	0	3	1
C	fieldSpecs()	0	32	7
A	fieldSpec()	0	4	2
A	tableClass()	0	18	4
B	cacheTableNames()	0	21	5
A	buildTableName()	0	11	2
A	databaseFields()	0	16	3
A	databaseField()	0	4	2
A	classHasTable()	0	4	1
A	compositeFields()	0	17	3
A	compositeField()	0	4	2
D	cacheDatabaseFields()	0	57	13
A	tableForField()	0	7	2
B	classForField()	0	23	5
B	manyManyComponent()	0	26	4
B	parseBelongsManyManyComponent()	0	24	4
B	manyManyExtraFieldsForComponent()	0	20	5
A	hasManyComponent()	0	14	3
A	hasOneComponent()	0	11	2
A	belongsToComponent()	0	14	3
B	parseManyManyComponent()	0	40	3
A	getManyManyInverseRelationship()	0	13	4
C	getRemoteJoinField()	0	68	12
C	checkManyManyFieldClass()	0	47	7
A	checkManyManyJoinClass()	0	16	3
C	checkRelationClass()	0	30	7

<?php

namespace SilverStripe\ORM;

use Exception;
use SilverStripe\Core\Injector\Injectable;
use SilverStripe\Core\Config\Configurable;
use SilverStripe\ORM\FieldType\DBComposite;
use SilverStripe\Core\ClassInfo;
use SilverStripe\Core\Config\Config;
use SilverStripe\Core\Object;
use InvalidArgumentException;
use LogicException;

/**
 * Provides dataobject and database schema mapping functionality
 */
class DataObjectSchema {

	use Injectable;
	use Configurable;

	/**
	 * Default separate for table namespaces. Can be set to any string for
	 * databases that do not support some characters.
	 *
	 * Defaults to \ to to conform to 3.x convention.
	 *
	 * @config
	 * @var string
	 */
	private static $table_namespace_separator = '\\';

	/**
	 * Cache of database fields
	 *
	 * @var array
	 */
	protected $databaseFields = [];

	/**
	 * Cache of composite database field
	 *
	 * @var array
	 */
	protected $compositeFields = [];

	/**
	 * Cache of table names
	 *
	 * @var array
	 */
	protected $tableNames = [];

	/**
	 * Clear cached table names
	 */
	public function reset() {
		$this->tableNames = [];
		$this->databaseFields = [];
		$this->compositeFields = [];
	}

	/**
	 * Get all table names
	 *
	 * @return array
	 */
	public function getTableNames() {
		$this->cacheTableNames();
		return $this->tableNames;
	}

	/**
	 * Given a DataObject class and a field on that class, determine the appropriate SQL for
	 * selecting / filtering on in a SQL string. Note that $class must be a valid class, not an
	 * arbitrary table.
	 *
	 * The result will be a standard ANSI-sql quoted string in "Table"."Column" format.
	 *
	 * @param string $class Class name (not a table).
	 * @param string $field Name of field that belongs to this class (or a parent class)
	 * @return string The SQL identifier string for the corresponding column for this field
	 */
	public function sqlColumnForField($class, $field) {
		$table = $this->tableForField($class, $field);
		if(!$table) {

The expression $table of type string|null is loosely compared to false; this is ambiguous if the string can be empty. You might want to explicitly use === null instead.

			throw new InvalidArgumentException("\"{$field}\" is not a field on class \"{$class}\"");
		}
		return "\"{$table}\".\"{$field}\"";
	}

	/**
	 * Get table name for the given class.
	 *
	 * Note that this does not confirm a table actually exists (or should exist), but returns
	 * the name that would be used if this table did exist.
	 *
	 * @param string $class
	 * @return string Returns the table name, or null if there is no table
	 */
	public function tableName($class) {
		$tables = $this->getTableNames();
		$class = ClassInfo::class_name($class);
		if(isset($tables[$class])) {
			return $tables[$class];
		}
		return null;
	}
	/**
	 * Returns the root class (the first to extend from DataObject) for the
	 * passed class.
	 *
	 * @param string|object $class
	 * @return string
	 * @throws InvalidArgumentException
	 */
	public function baseDataClass($class) {
		$class = ClassInfo::class_name($class);
		$current = $class;
		while ($next = get_parent_class($current)) {
			if ($next === DataObject::class) {
				return $current;
			}
			$current = $next;
		}
		throw new InvalidArgumentException("$class is not a subclass of DataObject");
	}

	/**
	 * Get the base table
	 *
	 * @param string|object $class
	 * @return string
	 */
	public function baseDataTable($class) {
		return $this->tableName($this->baseDataClass($class));
	}

	/**
	 * fieldSpec should exclude virtual fields (such as composite fields), and only include fields with a db column.
	 */
	const DB_ONLY = 1;

	/**
	 * fieldSpec should only return fields that belong to this table, and not any ancestors
	 */
	const UNINHERITED = 2;

	/**
	 * fieldSpec should prefix all field specifications with the class name in RecordClass.Column(spec) format.
	 */
	const INCLUDE_CLASS = 4;

	/**
	 * Get all DB field specifications for a class, including ancestors and composite fields.
	 *
	 * @param string|DataObject $classOrInstance
	 * @param int $options Bitmask of options
	 *  - UNINHERITED Limit to only this table
	 *  - DB_ONLY Exclude virtual fields (such as composite fields), and only include fields with a db column.
	 *  - INCLUDE_CLASS Prefix the field specification with the class name in RecordClass.Column(spec) format.
	 * @return array List of fields, where the key is the field name and the value is the field specification.
	 */
	public function fieldSpecs($classOrInstance, $options = 0) {
		$class = ClassInfo::class_name($classOrInstance);

		// Validate options
		if (!is_int($options)) {
			throw new InvalidArgumentException("Invalid options " . var_export($options, true));
		}
		$uninherited = ($options & self::UNINHERITED) === self::UNINHERITED;
		$dbOnly = ($options & self::DB_ONLY) === self::DB_ONLY;
		$includeClass = ($options & self::INCLUDE_CLASS) === self::INCLUDE_CLASS;

		// Walk class hierarchy
		$db = [];
		$classes = $uninherited ? [$class] : ClassInfo::ancestry($class);
		foreach($classes as $tableClass) {
			// Find all fields on this class
			$fields = $this->databaseFields($tableClass, false);

			// Merge with composite fields
			if (!$dbOnly) {
				$compositeFields = $this->compositeFields($tableClass, false);
				$fields = array_merge($fields, $compositeFields);
			}

			// Record specification
			foreach ($fields as $name => $specification) {
				$prefix = $includeClass ? "{$tableClass}." : "";
				$db[$name] =  $prefix . $specification;
			}
		}
		return $db;
	}


	/**
	 * Get specifications for a single class field
	 *
	 * @param string|DataObject $classOrInstance Name or instance of class
	 * @param string $fieldName Name of field to retrieve
	 * @param int $options Bitmask of options
	 *  - UNINHERITED Limit to only this table
	 *  - DB_ONLY Exclude virtual fields (such as composite fields), and only include fields with a db column.
	 *  - INCLUDE_CLASS Prefix the field specification with the class name in RecordClass.Column(spec) format.
	 * @return string|null Field will be a string in FieldClass(args) format, or
	 * RecordClass.FieldClass(args) format if using INCLUDE_CLASS. Will be null if no field is found.
	 */
	public function fieldSpec($classOrInstance, $fieldName, $options = 0) {
		$specs = $this->fieldSpecs($classOrInstance, $options);
		return isset($specs[$fieldName]) ? $specs[$fieldName] : null;
	}

	/**
	 * Find the class for the given table
	 *
	 * @param string $table
	 * @return string|null The FQN of the class, or null if not found
	 */
	public function tableClass($table) {
		$tables = $this->getTableNames();
		$class = array_search($table, $tables, true);
		if($class) {
			return $class;
		}

		// If there is no class for this table, strip table modifiers (e.g. _Live / _versions)
		// from the end and re-attempt a search.
		if(preg_match('/^(?<class>.+)(_[^_]+)$/i', $table, $matches)) {
			$table = $matches['class'];
			$class = array_search($table, $tables, true);
			if($class) {
				return $class;
			}
		}
		return null;
	}

	/**
	 * Cache all table names if necessary
	 */
	protected function cacheTableNames() {
		if($this->tableNames) {

The expression $this->tableNames of type array is implicitly converted to a boolean; are you sure this is intended? If so, consider using ! empty($expr) instead to make it clear that you intend to check for an array without elements.

			return;
		}
		$this->tableNames = [];
		foreach(ClassInfo::subclassesFor(DataObject::class) as $class) {
			if($class === DataObject::class) {
				continue;
			}
			$table = $this->buildTableName($class);

			// Check for conflicts
			$conflict = array_search($table, $this->tableNames, true);
			if($conflict) {
				throw new LogicException(
					"Multiple classes (\"{$class}\", \"{$conflict}\") map to the same table: \"{$table}\""
				);
			}
			$this->tableNames[$class] = $table;
		}
	}

	/**
	 * Generate table name for a class.
	 *
	 * Note: some DB schema have a hard limit on table name length. This is not enforced by this method.
	 * See dev/build errors for details in case of table name violation.
	 *
	 * @param string $class
	 * @return string
	 */
	protected function buildTableName($class) {
		$table = Config::inst()->get($class, 'table_name', Config::UNINHERITED);

		// Generate default table name
		if(!$table) {
			$separator = $this->config()->get('table_namespace_separator');
			$table = str_replace('\\', $separator, trim($class, '\\'));
		}

		return $table;
	}

	/**
	 * Return the complete map of fields to specification on this object, including fixed_fields.
	 * "ID" will be included on every table.
	 *
	 * @param string $class Class name to query from
	 * @param bool $aggregated Include fields in entire hierarchy, rather than just on this table
	 * @return array Map of fieldname to specification, similiar to {@link DataObject::$db}.
	 */
	public function databaseFields($class, $aggregated = true) {
		$class = ClassInfo::class_name($class);
		if($class === DataObject::class) {
			return [];
		}
		$this->cacheDatabaseFields($class);
		$fields = $this->databaseFields[$class];

		if (!$aggregated) {
			return $fields;
		}

		// Recursively merge
		$parentFields = $this->databaseFields(get_parent_class($class));
		return array_merge($fields, array_diff_key($parentFields, $fields));
	}

	/**
	 * Gets a single database field.
	 *
	 * @param string $class Class name to query from
	 * @param string $field Field name
	 * @param bool $aggregated Include fields in entire hierarchy, rather than just on this table
	 * @return string|null Field specification, or null if not a field
	 */
	public function databaseField($class, $field, $aggregated = true) {
		$fields = $this->databaseFields($class, $aggregated);
		return isset($fields[$field]) ? $fields[$field] : null;
	}

	/**
	 * Check if the given class has a table
	 *
	 * @param string $class
	 * @return bool
	 */
	public function classHasTable($class) {
		$fields = $this->databaseFields($class, false);
		return !empty($fields);
	}

	/**
	 * Returns a list of all the composite if the given db field on the class is a composite field.
	 * Will check all applicable ancestor classes and aggregate results.
	 *
	 * Can be called directly on an object. E.g. Member::composite_fields(), or Member::composite_fields(null, true)
	 * to aggregate.
	 *
	 * Includes composite has_one (Polymorphic) fields
	 *
	 * @param string $class Name of class to check
	 * @param bool $aggregated Include fields in entire hierarchy, rather than just on this table
	 * @return array List of composite fields and their class spec
	 */
	public function compositeFields($class, $aggregated = true) {
		$class = ClassInfo::class_name($class);
		if($class === DataObject::class) {
			return [];
		}
		$this->cacheDatabaseFields($class);

		// Get fields for this class
		$compositeFields = $this->compositeFields[$class];
		if(!$aggregated) {
			return $compositeFields;
		}

		// Recursively merge
		$parentFields = $this->compositeFields(get_parent_class($class));
		return array_merge($compositeFields, array_diff_key($parentFields, $compositeFields));
	}

	/**
	 * Get a composite field for a class
	 *
	 * @param string $class Class name to query from
	 * @param string $field Field name
	 * @param bool $aggregated Include fields in entire hierarchy, rather than just on this table
	 * @return string|null Field specification, or null if not a field
	 */
	public function compositeField($class, $field, $aggregated = true) {
		$fields = $this->compositeFields($class, $aggregated);
		return isset($fields[$field]) ? $fields[$field] : null;
	}

	/**
	 * Cache all database and composite fields for the given class.
	 * Will do nothing if already cached
	 *
	 * @param string $class Class name to cache
	 */
	protected function cacheDatabaseFields($class) {
		// Skip if already cached
		if (isset($this->databaseFields[$class]) && isset($this->compositeFields[$class])) {
			return;
		}
		$compositeFields = array();
		$dbFields = array();

		// Ensure fixed fields appear at the start
		$fixedFields = DataObject::config()->get('fixed_fields');
		if(get_parent_class($class) === DataObject::class) {
			// Merge fixed with ClassName spec and custom db fields
			$dbFields = $fixedFields;
		} else {
			$dbFields['ID'] = $fixedFields['ID'];
		}

		// Check each DB value as either a field or composite field
		$db = Config::inst()->get($class, 'db', Config::UNINHERITED) ?: array();
		foreach($db as $fieldName => $fieldSpec) {
			$fieldClass = strtok($fieldSpec, '(');
			if(singleton($fieldClass) instanceof DBComposite) {
				$compositeFields[$fieldName] = $fieldSpec;
			} else {
				$dbFields[$fieldName] = $fieldSpec;
			}
		}

		// Add in all has_ones
		$hasOne = Config::inst()->get($class, 'has_one', Config::UNINHERITED) ?: array();
		foreach($hasOne as $fieldName => $hasOneClass) {
			if($hasOneClass === DataObject::class) {
				$compositeFields[$fieldName] = 'PolymorphicForeignKey';
			} else {
				$dbFields["{$fieldName}ID"] = 'ForeignKey';
			}
		}

		// Merge composite fields into DB
		foreach($compositeFields as $fieldName => $fieldSpec) {
			$fieldObj = Object::create_from_string($fieldSpec, $fieldName);
			$fieldObj->setTable($class);
			$nestedFields = $fieldObj->compositeDatabaseFields();
			foreach($nestedFields as $nestedName => $nestedSpec) {
				$dbFields["{$fieldName}{$nestedName}"] = $nestedSpec;
			}
		}

		// Prevent field-less tables with only 'ID'
		if(count($dbFields) < 2) {
			$dbFields = [];
		}

		// Return cached results
		$this->databaseFields[$class] = $dbFields;
		$this->compositeFields[$class] = $compositeFields;
	}

	/**
	 * Returns the table name in the class hierarchy which contains a given
	 * field column for a {@link DataObject}. If the field does not exist, this
	 * will return null.
	 *
	 * @param string $candidateClass
	 * @param string $fieldName
	 * @return string
	 */
	public function tableForField($candidateClass, $fieldName) {
		$class = $this->classForField($candidateClass, $fieldName);
		if($class) {

The expression $class of type null|string is loosely compared to true; this is ambiguous if the string can be empty. You might want to explicitly use !== null instead.

			return $this->tableName($class);
		}
		return null;
	}

	/**
	 * Returns the class name in the class hierarchy which contains a given
	 * field column for a {@link DataObject}. If the field does not exist, this
	 * will return null.
	 *
	 * @param string $candidateClass
	 * @param string $fieldName
	 * @return string
	 */
	public function classForField($candidateClass, $fieldName)  {
		// normalise class name
		$candidateClass = ClassInfo::class_name($candidateClass);
		if($candidateClass === DataObject::class) {
			return null;
		}

		// Short circuit for fixed fields
		$fixed = DataObject::config()->get('fixed_fields');
		if(isset($fixed[$fieldName])) {
			return $this->baseDataClass($candidateClass);
		}

		// Find regular field
		while($candidateClass) {
			$fields = $this->databaseFields($candidateClass, false);
			if(isset($fields[$fieldName])) {
				return $candidateClass;
			}
			$candidateClass = get_parent_class($candidateClass);
		}
		return null;
	}

	/**
	 * Return information about a specific many_many component. Returns a numeric array.
	 * The first item in the array will be the class name of the relation.
	 *
	 * Standard many_many return type is:
	 *
	 * array(
	 *  <manyManyClass>,		Name of class for relation. E.g. "Categories"
	 * 	<classname>,			The class that relation is defined in e.g. "Product"
	 * 	<candidateName>,		The target class of the relation e.g. "Category"
	 * 	<parentField>,			The field name pointing to <classname>'s table e.g. "ProductID".
	 * 	<childField>,			The field name pointing to <candidatename>'s table e.g. "CategoryID".
	 * 	<joinTableOrRelation>	The join table between the two classes e.g. "Product_Categories".
	 *							If the class name is 'ManyManyThroughList' then this is the name of the
	 * 							has_many relation.
	 * )
	 * @param string $class Name of class to get component for
	 * @param string $component The component name
	 * @return array|null
	 */
	public function manyManyComponent($class, $component) {
		$classes = ClassInfo::ancestry($class);
		foreach($classes as $parentClass) {
			// Check if the component is defined in many_many on this class
			$manyMany = Config::inst()->get($parentClass, 'many_many', Config::UNINHERITED);
			if(isset($manyMany[$component])) {
				return $this->parseManyManyComponent($parentClass, $component, $manyMany[$component]);
			}

			// Check if the component is defined in belongs_many_many on this class
			$belongsManyMany = Config::inst()->get($parentClass, 'belongs_many_many', Config::UNINHERITED);
			if (!isset($belongsManyMany[$component])) {
				continue;
			}

			// Extract class and relation name from dot-notation
			list($childClass, $relationName)
				= $this->parseBelongsManyManyComponent($parentClass, $component, $belongsManyMany[$component]);

			// Build inverse relationship from other many_many, and swap parent/child
			list($relationClass, $childClass, $parentClass, $childField, $parentField, $joinTable)
				= $this->manyManyComponent($childClass, $relationName);
			return [$relationClass, $parentClass, $childClass, $parentField, $childField, $joinTable];
		}
		return null;
	}



	/**
	 * Parse a belongs_many_many component to extract class and relationship name
	 *
	 * @param string $parentClass Name of class
	 * @param string $component Name of relation on class
	 * @param string $specification specification for this belongs_many_many
	 * @return array Array with child class and relation name
	 */
	protected function parseBelongsManyManyComponent($parentClass, $component, $specification)
	{
		$childClass = $specification;
		$relationName = null;
		if (strpos($specification, '.') !== false) {
			list($childClass, $relationName) = explode('.', $specification, 2);
		}

		// We need to find the inverse component name, if not explicitly given
		if (!$relationName) {
			$relationName = $this->getManyManyInverseRelationship($childClass, $parentClass);
		}

		// Check valid relation found
		if (!$relationName) {
			throw new LogicException(
				"belongs_many_many relation {$parentClass}.{$component} points to "
				. "{$specification} without matching many_many"
			);
		}

		// Return relatios
		return array($childClass, $relationName);
	}

	/**
	 * Return the many-to-many extra fields specification for a specific component.
	 *
	 * @param string $class
	 * @param string $component
	 * @return array|null
	 */
	public function manyManyExtraFieldsForComponent($class, $component) {
		// Get directly declared many_many_extraFields
		$extraFields = Config::inst()->get($class, 'many_many_extraFields');
		if (isset($extraFields[$component])) {
			return $extraFields[$component];
		}

		// If not belongs_many_many then there are no components
		while ($class && ($class !== DataObject::class)) {
			$belongsManyMany = Config::inst()->get($class, 'belongs_many_many', Config::UNINHERITED);
			if (isset($belongsManyMany[$component])) {
				// Reverse relationship and find extrafields from child class
				list($childClass, $relationName) = $this->parseBelongsManyManyComponent($class, $component,
					$belongsManyMany[$component]);
				return $this->manyManyExtraFieldsForComponent($childClass, $relationName);
			}
			$class = get_parent_class($class);
		}
		return null;
	}

	/**
	 * Return data for a specific has_many component.
	 *
	 * @param string $class Parent class
	 * @param string $component
	 * @param bool $classOnly If this is TRUE, than any has_many relationships in the form
	 * "ClassName.Field" will have the field data stripped off. It defaults to TRUE.
	 * @return string|null
	 */
	public function hasManyComponent($class, $component, $classOnly = true) {
		$hasMany = (array)Config::inst()->get($class, 'has_many');
		if(!isset($hasMany[$component])) {
			return null;
		}

		// Remove has_one specifier if given
		$hasMany = $hasMany[$component];
		$hasManyClass = strtok($hasMany, '.');

		// Validate
		$this->checkRelationClass($class, $component, $hasManyClass, 'has_many');
		return $classOnly ? $hasManyClass : $hasMany;
	}

	/**
	 * Return data for a specific has_one component.
	 *
	 * @param string $class
	 * @param string $component
	 * @return string|null
	 */
	public function hasOneComponent($class, $component) {
		$hasOnes = Config::inst()->get($class, 'has_one');
		if(!isset($hasOnes[$component])) {
			return null;
		}

		// Validate
		$relationClass = $hasOnes[$component];
		$this->checkRelationClass($class, $component, $relationClass, 'has_one');
		return $relationClass;
	}

	/**
	 * Return data for a specific belongs_to component.
	 *
	 * @param string $class
	 * @param string $component
	 * @param bool $classOnly If this is TRUE, than any has_many relationships in the
	 * form "ClassName.Field" will have the field data stripped off. It defaults to TRUE.
	 * @return string|null
	 */
	public function belongsToComponent($class, $component, $classOnly = true) {
		$belongsTo = (array)Config::inst()->get($class, 'belongs_to');
		if(!isset($belongsTo[$component])) {
			return null;
		}

		// Remove has_one specifier if given
		$belongsTo = $belongsTo[$component];
		$belongsToClass = strtok($belongsTo, '.');

		// Validate
		$this->checkRelationClass($class, $component, $belongsToClass, 'belongs_to');
		return $classOnly ? $belongsToClass : $belongsTo;
	}

	/**
	 *
	 * @param string $parentClass Parent class name
	 * @param string $component ManyMany name
	 * @param string|array $specification Declaration of many_many relation type
	 * @return array
	 */
	protected function parseManyManyComponent($parentClass, $component, $specification)
	{
		// Check if this is many_many_through
		if (is_array($specification)) {
			// Validate join, parent and child classes
			$joinClass = $this->checkManyManyJoinClass($parentClass, $component, $specification);
			$parentClass = $this->checkManyManyFieldClass($parentClass, $component, $joinClass, $specification, 'from');
			$joinChildClass = $this->checkManyManyFieldClass($parentClass, $component, $joinClass, $specification, 'to');
			return [
				ManyManyThroughList::class,
				$parentClass,
				$joinChildClass,
				$specification['from'] . 'ID',
				$specification['to'] . 'ID',
				$joinClass,
			];
		}

		// Validate $specification class is valid
		$this->checkRelationClass($parentClass, $component, $specification, 'many_many');

		// automatic scaffolded many_many table
		$classTable = $this->tableName($parentClass);
		$parentField = "{$classTable}ID";
		if ($parentClass === $specification) {
			$childField = "ChildID";
		} else {
			$candidateTable = $this->tableName($specification);
			$childField = "{$candidateTable}ID";
		}
		$joinTable = "{$classTable}_{$component}";
		return [
			ManyManyList::class,
			$parentClass,
			$specification,
			$parentField,
			$childField,
			$joinTable,
		];
	}

	/**
	 * Find a many_many on the child class that points back to this many_many
	 *
	 * @param string $childClass
	 * @param string $parentClass
	 * @return string|null
	 */
	protected function getManyManyInverseRelationship($childClass, $parentClass)
	{
		$otherManyMany = Config::inst()->get($childClass, 'many_many', Config::UNINHERITED);
		if (!$otherManyMany) {
			return null;
		}
		foreach ($otherManyMany as $inverseComponentName => $nextClass) {
			if ($nextClass === $parentClass) {
				return $inverseComponentName;
			}
		}
		return null;
	}

	/**
	 * Tries to find the database key on another object that is used to store a
	 * relationship to this class. If no join field can be found it defaults to 'ParentID'.
	 *
	 * If the remote field is polymorphic then $polymorphic is set to true, and the return value
	 * is in the form 'Relation' instead of 'RelationID', referencing the composite DBField.
	 *
	 * @param string $class
	 * @param string $component Name of the relation on the current object pointing to the
	 * remote object.
	 * @param string $type the join type - either 'has_many' or 'belongs_to'
	 * @param boolean $polymorphic Flag set to true if the remote join field is polymorphic.
	 * @return string
	 * @throws Exception
	 */
	public function getRemoteJoinField($class, $component, $type = 'has_many', &$polymorphic = false) {
		// Extract relation from current object
		if($type === 'has_many') {
			$remoteClass = $this->hasManyComponent($class, $component, false);
		} else {
			$remoteClass = $this->belongsToComponent($class, $component, false);
		}

		if(empty($remoteClass)) {
			throw new Exception("Unknown $type component '$component' on class '$class'");
		}
		if(!ClassInfo::exists(strtok($remoteClass, '.'))) {
			throw new Exception(
				"Class '$remoteClass' not found, but used in $type component '$component' on class '$class'"
			);
		}

		// If presented with an explicit field name (using dot notation) then extract field name
		$remoteField = null;
		if(strpos($remoteClass, '.') !== false) {
			list($remoteClass, $remoteField) = explode('.', $remoteClass);
		}

		// Reference remote has_one to check against
		$remoteRelations = Config::inst()->get($remoteClass, 'has_one');

		// Without an explicit field name, attempt to match the first remote field
		// with the same type as the current class
		if(empty($remoteField)) {
			// look for remote has_one joins on this class or any parent classes
			$remoteRelationsMap = array_flip($remoteRelations);
			foreach(array_reverse(ClassInfo::ancestry($class)) as $class) {
				if(array_key_exists($class, $remoteRelationsMap)) {
					$remoteField = $remoteRelationsMap[$class];
					break;
				}
			}
		}

		// In case of an indeterminate remote field show an error
		if(empty($remoteField)) {
			$polymorphic = false;
			$message = "No has_one found on class '$remoteClass'";
			if($type == 'has_many') {
				// include a hint for has_many that is missing a has_one
				$message .= ", the has_many relation from '$class' to '$remoteClass'";
				$message .= " requires a has_one on '$remoteClass'";
			}
			throw new Exception($message);
		}

		// If given an explicit field name ensure the related class specifies this
		if(empty($remoteRelations[$remoteField])) {
			throw new Exception("Missing expected has_one named '$remoteField'
				on class '$remoteClass' referenced by $type named '$component'
				on class {$class}"
			);
		}

		// Inspect resulting found relation
		if($remoteRelations[$remoteField] === DataObject::class) {
			$polymorphic = true;
			return $remoteField; // Composite polymorphic field does not include 'ID' suffix
		} else {
			$polymorphic = false;
			return $remoteField . 'ID';
		}
	}

	/**
	 * Validate the to or from field on a has_many mapping class
	 *
	 * @param string $parentClass Name of parent class
	 * @param string $component Name of many_many component
	 * @param string $joinClass Class for the joined table
	 * @param array $specification Complete many_many specification
	 * @param string $key Name of key to check ('from' or 'to')
	 * @return string Class that matches the given relation
	 * @throws InvalidArgumentException
	 */
	protected function checkManyManyFieldClass($parentClass, $component, $joinClass, $specification, $key)
	{
		// Ensure value for this key exists
		if (empty($specification[$key])) {
			throw new InvalidArgumentException(
				"many_many relation {$parentClass}.{$component} has missing {$key} which "
				. "should be a has_one on class {$joinClass}"
			);
		}

		// Check that the field exists on the given object
		$relation = $specification[$key];
		$relationClass = $this->hasOneComponent($joinClass, $relation);
		if (empty($relationClass)) {
			throw new InvalidArgumentException(
				"many_many through relation {$parentClass}.{$component} {$key} references a field name "
				. "{$joinClass}::{$relation} which is not a has_one"
			);
		}

		// Check for polymorphic
		if ($relationClass === DataObject::class) {
			throw new InvalidArgumentException(
				"many_many through relation {$parentClass}.{$component} {$key} references a polymorphic field "
				. "{$joinClass}::{$relation} which is not supported"
			);
		}

		// Validate the join class isn't also the name of a field or relation on either side
		// of the relation
		$field = $this->fieldSpec($relationClass, $joinClass);
		if ($field) {

The expression $field of type string|null is loosely compared to true; this is ambiguous if the string can be empty. You might want to explicitly use !== null instead.

			throw new InvalidArgumentException(
				"many_many through relation {$parentClass}.{$component} {$key} class {$relationClass} "
				. " cannot have a db field of the same name of the join class {$joinClass}"
			);
		}

		// Validate bad types on parent relation
		if ($key === 'from' && $relationClass !== $parentClass) {
			throw new InvalidArgumentException(
				"many_many through relation {$parentClass}.{$component} {$key} references a field name "
				. "{$joinClass}::{$relation} of type {$relationClass}; {$parentClass} expected"
			);
		}
		return $relationClass;
	}

	/**
	 * @param string $parentClass Name of parent class
	 * @param string $component Name of many_many component
	 * @param array $specification Complete many_many specification
	 * @return string Name of join class
	 */
	protected function checkManyManyJoinClass($parentClass, $component, $specification)
	{
		if (empty($specification['through'])) {
			throw new InvalidArgumentException(
				"many_many relation {$parentClass}.{$component} has missing through which should be "
				. "a DataObject class name to be used as a join table"
			);
		}
		$joinClass = $specification['through'];
		if (!class_exists($joinClass)) {
			throw new InvalidArgumentException(
				"many_many relation {$parentClass}.{$component} has through class \"{$joinClass}\" which does not exist"
			);
		}
		return $joinClass;
	}

	/**
	 * Validate a given class is valid for a relation
	 *
	 * @param string $class Parent class
	 * @param string $component Component name
	 * @param string $relationClass Candidate class to check
	 * @param string $type Relation type (e.g. has_one)
	 */
	protected function checkRelationClass($class, $component, $relationClass, $type)
	{
		if (!is_string($component) || is_numeric($component)) {
			throw new InvalidArgumentException(
				"{$class} has invalid {$type} relation name"
			);
		}
		if (!is_string($relationClass)) {
			throw new InvalidArgumentException(
				"{$type} relation {$class}.{$component} is not a class name"
			);
		}
		if (!class_exists($relationClass)) {
			throw new InvalidArgumentException(
				"{$type} relation {$class}.{$component} references class {$relationClass} which doesn't exist"
			);
		}
		// Support polymorphic has_one
		if ($type === 'has_one') {
			$valid = is_a($relationClass, DataObject::class, true);
		} else {
			$valid = is_subclass_of($relationClass, DataObject::class, true);
		}
		if (!$valid) {
			throw new InvalidArgumentException(
				"{$type} relation {$class}.{$component} references class {$relationClass} "
				. " which is not a subclass of " . DataObject::class
			);
		}
	}
}