Filtering Get_terms Query For Custom Taxonomies In WordPress
When working with WordPress, custom taxonomies provide a powerful way to organize and categorize your content beyond the standard categories and tags. The get_terms function is a fundamental tool for retrieving terms within these taxonomies. However, in many scenarios, you'll need to filter the results of get_terms to retrieve a specific subset of terms. This article dives deep into how to effectively filter get_terms queries, providing practical examples and addressing common use cases to enhance your WordPress development skills.
Understanding get_terms and Its Parameters
The get_terms function in WordPress is used to retrieve an array of terms from a specified taxonomy. Before we delve into filtering, it's crucial to understand the basic usage and available parameters. The most basic implementation involves passing an array of arguments to the function, with the taxonomy argument being the most essential. Let's break down the key parameters you can use to tailor your queries.
Core Parameters
- taxonomy (string|string[]) (Required): This is the most crucial parameter, specifying the taxonomy or taxonomies from which to retrieve terms. You can pass a single taxonomy name as a string or an array of taxonomy names to fetch terms from multiple taxonomies simultaneously.
- hide_empty (bool) (Optional): Determines whether to include terms that have no associated posts. Setting it to
true(as in the initial problem) excludes empty terms, whilefalseincludes them. This parameter is essential for cleaning up your term lists and presenting only relevant categories or tags to your users. - orderby (string) (Optional): Specifies the sorting order for the returned terms. Common values include
name,slug,term_group,term_id,id,description, andcount. The default isname, which sorts terms alphabetically. - order (string) (Optional): Works in conjunction with
orderbyto define the sort direction. It can be eitherASC(ascending) orDESC(descending). The default isASC. - number (int|string) (Optional): Limits the number of terms to return. If set to a positive integer,
get_termswill return at most that many terms. If set to0or left empty, it returns all terms matching the other criteria. This is extremely useful for pagination or displaying a limited set of featured terms. - offset (int) (Optional): Specifies the number of terms to skip before starting to return terms. This is often used in conjunction with
numberfor pagination, allowing you to fetch terms in batches. - slug (string|string[]) (Optional): Retrieves terms with specific slugs. You can pass a single slug as a string or an array of slugs to retrieve multiple terms by their slugs. This is particularly useful when you need to fetch specific terms programmatically without relying on their IDs.
- name (string|string[]) (Optional): Retrieves terms with specific names. Similar to
slug, you can pass a single name or an array of names. Note that term names are not necessarily unique, so this parameter might return multiple terms if they share the same name in different taxonomies. - term_taxonomy_id (int|int[]) (Optional): Retrieves terms with specific term taxonomy IDs. This is useful when you need to target specific relationships between terms and taxonomies.
- object_ids (int|int[]) (Optional): Retrieves terms associated with specific posts or objects. Pass an array of post IDs, and
get_termswill return only the terms associated with those posts. This is particularly powerful for creating dynamic term lists based on the content being displayed. - search (string) (Optional): Searches for terms whose names or slugs contain the specified string. This is useful for implementing term search functionality within your WordPress site.
- cache_domain (string) (Optional): The domain to use for caching the query results. This is an advanced parameter that can improve performance by reducing database queries.
- pad_counts (bool) (Optional): Recalculates term counts, including counts for child terms. This ensures accurate counts when displaying hierarchical taxonomies.
- child_of (int) (Optional): Retrieves terms that are children of a specific term ID. This is essential for building hierarchical term lists or navigation menus.
- parent (int) (Optional): Retrieves terms that are direct children of a specific term ID. Unlike
child_of, this only retrieves immediate children, not descendants. - childless (bool) (Optional): If set to
true, retrieves only terms that have no children. - hierarchical (bool) (Optional): Whether to retrieve terms in a hierarchical order. This is particularly useful for displaying terms in a tree-like structure.
- meta_query (array) (Optional): A powerful parameter that allows you to filter terms based on their meta values. This opens up a wide range of possibilities for advanced filtering, such as retrieving terms with specific custom field values.
Example of Basic get_terms Usage
Before diving into more complex filtering techniques, let's illustrate a basic example of using get_terms:
$terms = get_terms( array(
'taxonomy' => 'my_custom_taxonomy',
'hide_empty' => true,
) );
if ( ! empty( $terms ) && ! is_wp_error( $terms ) ) {
echo '<ul>';
foreach ( $terms as $term ) {
echo '<li><a href="' . esc_url( get_term_link( $term ) ) . '">' . esc_html( $term->name ) . '</a></li>';
}
echo '</ul>';
}
This code snippet retrieves all non-empty terms from the my_custom_taxonomy taxonomy and displays them as a list of links. Now, let's explore how to filter these results to meet more specific requirements.
Filtering get_terms with Specific Parameters
The power of get_terms lies in its ability to be filtered using various parameters. Let's explore some common filtering scenarios.
Filtering by Slug or Name
Sometimes, you need to retrieve specific terms based on their slug or name. The slug and name parameters allow you to do just that. For instance, if you need to fetch a term with the slug featured-product, you would use the following:
$terms = get_terms( array(
'taxonomy' => 'product_category',
'slug' => 'featured-product',
) );
if ( ! empty( $terms ) && ! is_wp_error( $terms ) ) {
$term = $terms[0]; // Assuming only one term with this slug
echo 'Term Name: ' . esc_html( $term->name );
}
Similarly, to fetch terms by name, you can use the name parameter:
$terms = get_terms( array(
'taxonomy' => 'product_category',
'name' => 'Featured Product',
) );
Filtering by Parent Term
For hierarchical taxonomies, filtering by parent term is a common requirement. The child_of and parent parameters allow you to retrieve child terms of a specific parent. The child_of parameter retrieves all descendants, while parent retrieves only immediate children. For example:
$terms = get_terms( array(
'taxonomy' => 'category',
'child_of' => 5, // Retrieve all categories that are children of category ID 5
) );
To retrieve only immediate children, use the parent parameter:
$terms = get_terms( array(
'taxonomy' => 'category',
'parent' => 5, // Retrieve only the immediate children of category ID 5
) );
Limiting the Number of Terms
The number parameter is essential for pagination or displaying a limited set of terms. For instance, to retrieve only the first 5 terms:
$terms = get_terms( array(
'taxonomy' => 'product_category',
'number' => 5,
) );
Excluding Terms
Sometimes, you need to exclude specific terms from the results. While get_terms doesn't have a direct exclude parameter, you can achieve this using the exclude parameter in conjunction with wp_list_categories or by manually filtering the results.
For example, using wp_list_categories:
wp_list_categories( array(
'taxonomy' => 'category',
'exclude' => array( 1, 2, 3 ), // Exclude categories with IDs 1, 2, and 3
) );
Alternatively, you can manually filter the results after using get_terms:
$terms = get_terms( array(
'taxonomy' => 'category',
) );
$excluded_ids = array( 1, 2, 3 );
$filtered_terms = array_filter( $terms, function( $term ) use ( $excluded_ids ) {
return ! in_array( $term->term_id, $excluded_ids );
} );
Advanced Filtering with meta_query
The meta_query parameter is where get_terms truly shines in terms of advanced filtering. It allows you to filter terms based on their meta values, opening up a wide range of possibilities. Let's explore how meta_query works and some practical examples.
Understanding meta_query Structure
The meta_query parameter accepts an array of arguments, where each argument represents a meta query clause. Each clause consists of the following keys:
- key (string) (Required): The meta key to query.
- value (mixed) (Optional): The meta value to compare against. Can be a single value or an array of values.
- compare (string) (Optional): The comparison operator. Common values include
=,!=,>,>=,<,<=,LIKE,NOT LIKE,IN,NOT IN,BETWEEN,NOT BETWEEN,EXISTS, andNOT EXISTS. The default is=. For numeric values, useNUMERICas thetype. - type (string) (Optional): The meta value type. Common values include
NUMERIC,BINARY,CHAR,DATE,DATETIME,DECIMAL,SIGNED,TIME, andUNSIGNED. The default isCHAR.
Multiple clauses can be combined using relation, which can be either AND or OR. The default is AND.
Example: Filtering by Meta Value
Suppose you have a custom taxonomy called product_category and you've added a meta field called is_featured to indicate whether a category is featured. To retrieve only the featured categories, you can use meta_query like this:
$terms = get_terms( array(
'taxonomy' => 'product_category',
'meta_query' => array(
array(
'key' => 'is_featured',
'value' => '1',
'compare' => '=',
'type' => 'NUMERIC', // Ensure numeric comparison
),
),
) );
This code snippet retrieves terms where the is_featured meta field is equal to 1. The type parameter is set to NUMERIC to ensure correct comparison for numeric values.
Combining Multiple Meta Queries
You can combine multiple meta queries using the relation parameter. For example, to retrieve categories that are both featured and have a minimum product count, you can use:
$terms = get_terms( array(
'taxonomy' => 'product_category',
'meta_query' => array(
'relation' => 'AND',
array(
'key' => 'is_featured',
'value' => '1',
'compare' => '=',
'type' => 'NUMERIC',
),
array(
'key' => 'product_count',
'value' => 10,
'compare' => '>=',
'type' => 'NUMERIC',
),
),
) );
This code snippet retrieves terms that have is_featured equal to 1 and product_count greater than or equal to 10.
To retrieve categories that are either featured or have a high product count, you can change the relation to OR:
$terms = get_terms( array(
'taxonomy' => 'product_category',
'meta_query' => array(
'relation' => 'OR',
array(
'key' => 'is_featured',
'value' => '1',
'compare' => '=',
'type' => 'NUMERIC',
),
array(
'key' => 'product_count',
'value' => 10,
'compare' => '>=',
'type' => 'NUMERIC',
),
),
) );
Using IN and NOT IN Operators
The IN and NOT IN operators are useful for querying meta values against an array of values. For example, to retrieve categories with specific promotion codes:
$terms = get_terms( array(
'taxonomy' => 'product_category',
'meta_query' => array(
array(
'key' => 'promotion_code',
'value' => array( 'SUMMER20', 'WINTER20' ),
'compare' => 'IN',
),
),
) );
This retrieves terms where the promotion_code meta field is either SUMMER20 or WINTER20.
Filtering by Meta Existence
The EXISTS and NOT EXISTS operators allow you to filter terms based on whether a meta key exists. For example, to retrieve categories that have a promotion_code meta field:
$terms = get_terms( array(
'taxonomy' => 'product_category',
'meta_query' => array(
array(
'key' => 'promotion_code',
'compare' => 'EXISTS',
),
),
) );
Practical Use Cases and Examples
To further illustrate the power of filtering get_terms, let's explore some practical use cases.
Displaying Featured Categories
As demonstrated earlier, you can use meta_query to retrieve and display featured categories. This is a common requirement for e-commerce sites or any site that wants to highlight specific categories.
$featured_categories = get_terms( array(
'taxonomy' => 'product_category',
'meta_query' => array(
array(
'key' => 'is_featured',
'value' => '1',
'compare' => '=',
'type' => 'NUMERIC',
),
),
) );
if ( ! empty( $featured_categories ) && ! is_wp_error( $featured_categories ) ) {
echo '<h2>Featured Categories</h2>';
echo '<ul>';
foreach ( $featured_categories as $category ) {
echo '<li><a href="' . esc_url( get_term_link( $category ) ) . '">' . esc_html( $category->name ) . '</a></li>';
}
echo '</ul>';
}
Creating a Category Filter for a Product Listing
On an e-commerce site, you might want to allow users to filter products by category. You can use get_terms to retrieve the available categories and display them as a filter.
$product_categories = get_terms( array(
'taxonomy' => 'product_category',
'hide_empty' => true,
) );
if ( ! empty( $product_categories ) && ! is_wp_error( $product_categories ) ) {
echo '<form method="get">';
echo '<label for="product_category">Filter by Category:</label>';
echo '<select name="product_category" id="product_category">';
echo '<option value="">All Categories</option>';
foreach ( $product_categories as $category ) {
echo '<option value="' . esc_attr( $category->slug ) . '">' . esc_html( $category->name ) . '</option>';
}
echo '</select>';
echo '<input type="submit" value="Filter">';
echo '</form>';
}
This code snippet generates a dropdown select element populated with product categories. When the user selects a category and submits the form, you can use the selected category slug to filter the product query.
Displaying Terms with Specific Custom Fields
You might have scenarios where you need to display terms based on specific custom field values. For instance, you might want to display a list of authors with their job titles.
$authors = get_terms( array(
'taxonomy' => 'author',
'meta_query' => array(
array(
'key' => 'job_title',
'compare' => 'EXISTS',
),
),
) );
if ( ! empty( $authors ) && ! is_wp_error( $authors ) ) {
echo '<h2>Authors</h2>';
echo '<ul>';
foreach ( $authors as $author ) {
$job_title = get_term_meta( $author->term_id, 'job_title', true );
echo '<li>' . esc_html( $author->name ) . ' - ' . esc_html( $job_title ) . '</li>';
}
echo '</ul>';
}
This code snippet retrieves authors who have a job_title meta field and displays their names along with their job titles.
Best Practices for Filtering get_terms
To ensure efficient and maintainable code when filtering get_terms, consider the following best practices:
- Use Specific Parameters: Whenever possible, use specific parameters like
slug,name, orterm_idto directly target the terms you need. This is more efficient than retrieving all terms and then filtering them manually. - Optimize
meta_query: When usingmeta_query, ensure that your queries are optimized. Use the correcttypeandcomparevalues, and avoid complex queries that can impact performance. - Cache Results: For frequently used queries, consider caching the results to reduce database load. WordPress provides various caching mechanisms that you can leverage.
- Use Transients: Transients are a simple way to store and retrieve cached data. You can use transients to cache the results of
get_termsqueries for a specified period. - Code Readability: Write clear and well-documented code. Use meaningful variable names and add comments to explain complex filtering logic.
Common Pitfalls and How to Avoid Them
While filtering get_terms is powerful, there are some common pitfalls to watch out for:
- Performance Issues: Overly complex queries, especially with
meta_query, can lead to performance issues. Optimize your queries and consider caching. - Incorrect Meta Types: Using the wrong
typeinmeta_querycan lead to incorrect results. Ensure you use the appropriate type for your meta values (e.g.,NUMERICfor numbers). - Typos in Meta Keys: Typos in meta keys are a common source of errors. Double-check your meta keys to ensure they are correct.
- Forgetting to Clear Cache: When updating terms or meta values, remember to clear any cached results to ensure you're displaying the latest data.
Conclusion
Filtering get_terms is a crucial skill for WordPress developers working with custom taxonomies. By understanding the available parameters and mastering techniques like meta_query, you can efficiently retrieve and display terms based on your specific requirements. Whether you're building a complex e-commerce site or a simple blog, the ability to filter terms effectively will significantly enhance your WordPress development capabilities. Remember to follow best practices, optimize your queries, and avoid common pitfalls to ensure your code is efficient, maintainable, and performs well.
By implementing these strategies, you can create dynamic and engaging WordPress sites that provide a seamless user experience.