Skip to content

Retrieving resources

Jump to bottom
E. Lynette Rayle edited this page Oct 23, 2020 · 11 revisions

back to Introduction

Goals

  • Using a query service:
    • Find a resource by its ID.
    • Find a resource by an alternate ID.
    • Find all resources of a specific model.
    • Find all members of a parent resource, and the parent of a member resource.
  • Understand other relationships.
  • Understand custom queries.

Reference: Valkyrie Query Documentation | Valkyrie Custom Query Documentation | postgres query_service | shared specs

Terminology

  • query service - provides methods for the standard set of queries for finding resources
  • custom query - additional query methods written in your app following the custom query pattern that are not part of the standard set of queries

Characteristics

All the examples in this section assume that you completed the examples under Saving a resource. If you went beyond the examples, you may see additional results beyond those shown for each example.

Finding a resource by id

To find the book by its id, you can either pass the string representation of the ID or the instance of Valkyrie::ID.

Substitute the actual id assigned during [[Saving a resource] for AN_ID_ASSIGNED_BY_POSTGRES.

> book = ValkyriePgDemo.pg_query_service.find_by(id: '_AN_ID_ASSIGNED_BY_POSTGRES_')
> book.id
  => #<Valkyrie::ID:... @id="_AN_ID_ASSIGNED_BY_POSTGRES_">
> same_book = ValkyriePgDemo.pg_query_service.find_by(id: book.id)
> book == same_book
  => true
> book.persisted?   
  => true
> book.title
  => ['Free Fall']
> book.title = "Lullaby Town"
> book.title
  => ["Lullaby Town"]
> book.persisted?
  => true # persisted? indicates only that the resource is known to exist in the database
> book == same_book
  => false # since the title was changed in book, but not same_book

NOTE: Since book with the new title wasn't persisted again (even though the persisted? method is returning true), the database continues to have the original title ['Free Fall'].

Finding a resource by alternate_ids

alternate_ids is a special attribute name which the query service knows about. There are methods defined to retrieve resources by an alternate id.

The book resource was saved with alternate_ids [#<Valkyrie::ID:0x00007fbf9c08e770 @id="b1">].

> book = ValkyriePgDemo.pg_query_service.find_by_alternate_identifier(alternate_identifier: 'b1')
> book.id
  => #<Valkyrie::ID:... @id="_AN_ID_ASSIGNED_BY_POSTGRES_">
> book.title
  => ["Free Fall"]

Finding all resources by model

To find all books, get all resources for a model.

> books = ValkyriePgDemo.pg_query_service.find_all_of_model(model: Book)
> books.each { |b| puts("class: #{b.class}    title: #{b.title}") }        
  => class: Book    title: ["Free Fall"]

Finding based on member_ids relationship

member_ids is a special attribute name which the query service knows about. There are methods defined to retrieve members and their parents.
warning_16 If you've been playing around a bit, your objects for book and page may not have membership as expected. You can look in the database for book with members. Use find_by(id:) described above to set book. You can do something similar for page. 
> child_pages = ValkyriePgDemo.pg_query_service.find_members(resource: book)
> child_pages.each { |p| puts("class: #{p.class}    page_num: #{p.page_num}    structure: #{p.structure}") }
class: Page    page_num: 1    structure: title page
  => nil
> parent_books = ValkyriePgDemo.pg_query_service.find_parents(resource: page)
> parent_books.each { |b| puts("class: #{b.class}    title: #{b.title}") }        
class: Book    title: ["Free Fall"]
  => nil

Finding based on other relationships

There are methods to find other relationships between resources that are maintained in attributes other than member_ids. See #find_references_by and #find_inverse_references in the shared specs for details on how they work.

List of all standard query methods

You can see the list of all queries that each data source adapter should support in shared_specs/queries

Custom Queries

Reference: Custom queries in Valkyrie wiki documentation

You may want to add additional queries beyond the standard set of queries. Some reasons why you may want to implement custom queries:

  • complex query that can be completed with the standard set of queries and is used in multiple places in the app
  • simple or complex queries that cannot be completed with the standard set of queries. NOTE: In this case, you will need to implement these queries for each adapter your app supports.

Previous | Next

Clone this wiki locally