Module: OAI::Provider

Included in:
Base
Defined in:
lib/oai/provider.rb,
lib/oai/provider/model.rb,
lib/oai/provider/response.rb,
lib/oai/provider/partial_result.rb,
lib/oai/provider/resumption_token.rb,
lib/oai/provider/model/activerecord_wrapper.rb,
lib/oai/provider/model/activerecord_caching_wrapper.rb

Overview

# OAI::Provider

Open Archives Initiative - Protocol for Metadata Harvesting see <www.openarchives.org/>

## Features

  • Easily setup a simple repository

  • Simple integration with ActiveRecord

  • Dublin Core metadata format included

  • Easily add addition metadata formats

  • Adaptable to any data source

  • Simple resumption token support

## Usage

To create a functional provider either subclass Base, or reconfigure the defaults.

### Sub classing a provider

“‘ruby

class MyProvider < Oai::Provider
  repository_name 'My little OAI provider'
  repository_url  'http://localhost/provider'
  record_prefix 'oai:localhost'
  admin_email 'root@localhost'   # String or Array
  source_model MyModel.new       # Subclass of OAI::Provider::Model
end

“‘

### Configuring the default provider

“‘ruby

class Oai::Provider::Base
  repository_name 'My little OAI Provider'
  repository_url 'http://localhost/provider'
  record_prefix 'oai:localhost'
  admin_email 'root@localhost'
  # record_prefix will be automatically prepended to sample_id, so in this
  # case it becomes: oai:localhost:13900
  sample_id '13900'
  source_model MyModel.new
end

“‘

The provider does allow a URL to be passed in at request processing time in case the repository URL cannot be determined ahead of time.

## Integrating with frameworks

### Camping

In the Models module of your camping application post model definition:

“‘ruby

class CampingProvider < OAI::Provider::Base
  repository_name 'Camping Test OAI Repository'
  source_model ActiveRecordWrapper.new(YOUR_ACTIVE_RECORD_MODEL)
end

“‘

In the Controllers module:

“‘ruby

class Oai
  def get
    @headers['Content-Type'] = 'text/xml'
    provider = Models::CampingProvider.new
    provider.process_request(@input.merge(:url => "http:"+URL(Oai).to_s))
  end
end

“‘

The provider will be available at “/oai”

### Rails

At the bottom of environment.rb create a OAI Provider:

“‘ruby

# forgive the standard blog example.

require 'oai'
class BlogProvider < OAI::Provider::Base
  repository_name 'My little OAI Provider'
  repository_url 'http://localhost:3000/provider'
  record_prefix 'oai:blog'
  admin_email 'root@localhost'
  source_model OAI::Provider::ActiveRecordWrapper.new(Post)
  sample_id '13900' # record prefix used, so becomes oai:blog:13900
end

“‘

Create a custom controller:

“‘ruby

class OaiController < ApplicationController
  def index
    provider = BlogProvider.new
    response =  provider.process_request(oai_params.to_h)
    render :body => response, :content_type => 'text/xml'
  end

  private

  def oai_params
    params.permit(:verb, :identifier, :metadataPrefix, :set, :from, :until, :resumptionToken)
  end
end

“‘

And route to it in your ‘config/routes.rb` file:

“‘ruby

match 'oai', to: "oai#index", via: [:get, :post]

“‘

Special thanks to Jose Hales-Garcia for this solution.

### Leverage the Provider instance

The traditional implementation of the OAI::Provider would pass the OAI::Provider class to the different resposnes. This made it hard to inject context into a common provider. Consider that we might have different request headers that change the scope of the OAI::Provider queries.

“‘ruby class InstanceProvider

def initialize(options = {})
  super({ :provider_context => :instance_based })
  @controller = options.fetch(:controller)
end
attr_reader :controller

end

class OaiController < ApplicationController

def index
  provider = InstanceProvider.new({ :controller => self })
  request_body =  provider.process_request(oai_params.to_h)
  render :body => request_body, :content_type => 'text/xml'
end

“‘

In the above example, the underlying response object will now receive an instance of the InstanceProvider. Without the ‘super({ :provider_context => :instance_based })` the response objects would have received the class InstanceProvider as the given provider.

## Supporting custom metadata formats

See MetadataFormat for details.

## ActiveRecord Integration

ActiveRecord integration is provided by the ‘ActiveRecordWrapper` class. It takes one required paramater, the class name of the AR class to wrap, and optional hash of options.

As of ‘oai` gem 1.0.0, Rails 5.2.x and Rails 6.0.x are supported. Please check the .travis.yml file at root of repo to see what versions of ruby/rails are being tested, in case this is out of date.

Valid options include:

  • ‘timestamp_field` - Specifies the model field/method to use as the update filter. Defaults to `updated_at`.

  • ‘identifier_field` – specifies the model field/method to use to get value to use

    as oai identifier (method return value should not include prefix)

  • ‘limit` - Maximum number of records to return in each page/set. Defaults to 100, set to `nil` for all records in one page. Otherwise the wrapper will paginate the result via resumption tokens. _Caution: specifying too large a limit will adversely affect performance._

Mapping from a ActiveRecord object to a specific metadata format follows this set of rules:

  1. Does ‘Model#to_metadata_prefix` exist? If so just return the result.

  2. Does the model provide a map via ‘Model.map_metadata_prefix`? If so use the map to generate the xml document.

  3. Loop thru the fields of the metadata format and check to see if the model responds to either the plural, or singular of the field.

For maximum control of the xml metadata generated, it’s usually best to provide a ‘to_metadata_prefix` in the model. If using Builder be sure not to include any `instruct!` in the xml object.

### Explicit creation example

“‘ruby

class Post < ActiveRecord::Base
  def to_oai_dc
    xml = Builder::XmlMarkup.new
    xml.tag!("oai_dc:dc",
      'xmlns:oai_dc' => "http://www.openarchives.org/OAI/2.0/oai_dc/",
      'xmlns:dc' => "http://purl.org/dc/elements/1.1/",
      'xmlns:xsi' => "http://www.w3.org/2001/XMLSchema-instance",
      'xsi:schemaLocation' =>
        %{http://www.openarchives.org/OAI/2.0/oai_dc/
          http://www.openarchives.org/OAI/2.0/oai_dc.xsd}) do
        xml.tag!('oai_dc:title', title)
        xml.tag!('oai_dc:description', text)
        xml.tag!('oai_dc:creator', user)
        tags.each do |tag|
          xml.tag!('oai_dc:subject', tag)
        end
    end
    xml.target!
  end
end

“‘

### Mapping Example

“‘ruby

# Extremely contrived mapping
class Post < ActiveRecord::Base
  def self.map_oai_dc
    {:subject => :tags,
     :description => :text,
     :creator => :user,
     :contibutor => :comments}
  end
end

“‘ ### Scopes for restrictions or eager-loading

Instead of passing in a Model class to OAI::Provider::ActiveRecordWrapper, you can actually pass in any scope (or ActiveRecord::Relation). This means you can use it for restrictions:

OAI::Provider::ActiveRecordWrapper.new(Post.where(published: true))

Or eager-loading an association you will need to create serialization, to avoid n+1 query performance problems:

OAI::Provider::ActiveRecordWrapper.new(Post.includes(:categories))

Or both of those in combination, or anything else that returns an ActiveRecord::Relation, including using custom scopes, etc.

### Sets?

There is some code written to support oai-pmh “sets” in the ActiveRecord::Wrapper, but it’s somewhat inflexible, and not well-documented, and as I write this I don’t understand it enough to say more. See github.com/code4lib/ruby-oai/issues/67

Defined Under Namespace

Modules: Metadata, Response Classes: ActiveRecordCachingWrapper, ActiveRecordWrapper, Base, Model, OaiEntry, OaiToken, PartialResult, ResumptionToken