Class: GraphQL::Models::RelationLoader

Inherits:
Batch::Loader
  • Object
show all
Defined in:
lib/graphql/models/relation_loader.rb

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(model_class) ⇒ RelationLoader

Returns a new instance of RelationLoader.



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# File 'lib/graphql/models/relation_loader.rb', line 7

def initialize(model_class)
  @model_class = model_class
end

Instance Attribute Details

#model_class ⇒ Object (readonly)

Returns the value of attribute model_class.



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# File 'lib/graphql/models/relation_loader.rb', line 5

def model_class
  @model_class
end

Instance Method Details

#fulfill_request(request, result) ⇒ Object 



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# File 'lib/graphql/models/relation_loader.rb', line 97

def fulfill_request(request, result)
  result = request.ensure_cardinality(result)
  request.fulfilled(result)
  fulfill(request, result)
end

#perform(load_requests) ⇒ Object 



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# File 'lib/graphql/models/relation_loader.rb', line 11

def perform(load_requests)
  # Group the requests to load by id into a single relation, and we'll fan it back out after
  # we have the results

  relations = []
  id_requests = load_requests.select { |r| r.load_type == :id }
  id_relation = model_class.where(id: id_requests.map(&:load_target))

  relations.push(id_relation) if id_requests.any?
  relations_to_requests = {}

  # Gather up all of the requests to load a relation, and map the relations back to their requests
  load_requests.select { |r| r.load_type == :relation }.each do |request|
    relation = request.load_target
    relations.push(relation) unless relations.detect { |r| r.object_id == relation.object_id }
    relations_to_requests[relation.object_id] ||= []
    relations_to_requests[relation.object_id].push(request)
  end

  # We need to build a query that will return all of the rows that match any of the relations.
  # But in addition, we also need to know how that relation sorted them. So we pull any ordering
  # information by adding RANK() columns to the query, and we determine whether that row belonged
  # to the query by adding CASE columns.

  # Map each relation to a SQL query that selects only the ID column for the rows that match it
  selection_clauses = relations.map do |relation|
    relation.unscope(:select).select(model_class.primary_key).to_sql
  end

  # Generate a CASE column that will tell us whether the row matches this particular relation
  slicing_columns = relations.each_with_index.map do |_relation, index|
    %{ CASE WHEN "#{model_class.table_name}"."#{model_class.primary_key}" IN (#{selection_clauses[index]}) THEN 1 ELSE 0 END AS "in_relation_#{index}" }
  end

  # For relations that have sorting applied, generate a RANK() column that tells us how the rows are
  # sorted within that relation
  sorting_columns = relations.each_with_index.map do |relation, index|
    arel = relation.arel
    next nil unless arel.orders.any?

    order_by = arel.orders.map do |expr|
      if expr.is_a?(Arel::Nodes::SqlLiteral)
        expr.to_s
      else
        expr.to_sql
      end
    end

    %{ RANK() OVER (ORDER BY #{order_by.join(', ')}) AS "sort_relation_#{index}" }
  end

  sorting_columns.compact!

  # Build the query that will select any of the rows that match the selection clauses
  main_relation = model_class
    .where("id in ((#{selection_clauses.join(") UNION (")}))")
    .select(%( "#{model_class.table_name}".* ))

  main_relation = slicing_columns.reduce(main_relation) { |relation, memo| relation.select(memo) }
  main_relation = sorting_columns.reduce(main_relation) { |relation, memo| relation.select(memo) }

  # Run the query
  result = main_relation.to_a

  # Now multiplex the results out to all of the relations that had asked for values
  relations.each_with_index do |relation, index|
    slice_col = "in_relation_#{index}"
    sort_col = "sort_relation_#{index}"

    matching_rows = result.select { |r| r[slice_col] == 1 }.sort_by { |r| r[sort_col] }

    if relation.object_id == id_relation.object_id
      pk = relation.klass.primary_key

      id_requests.each do |request|
        row = matching_rows.detect { |r| r[pk] == request.load_target }
        fulfill(request, row)
      end
    else
      relations_to_requests[relation.object_id].each do |request|
        fulfill_request(request, matching_rows)
      end
    end
  end
end