以编程方式预加载has_many,通过:在Elixir Ecto中

Question

我正在尝试以编程方式将预加载附加到我的某个具有has_many, through:关系的模型的查询中.

我的模块:

defmodule MyApp.Chemical do
  use MyApp.Web, :model

  schema "chemicals" do
    has_many :company_chemicals, MyApp.CompanyChemical
    has_many :companies, through: [:company_chemicals, :companies]

    field :name, :string
  end

  def with_companies(query) do
          from  chem      in query,
     left_join: comp_chem in assoc(chem, :company_chemicals),
          join: company   in assoc(comp_chem, :company),
       preload: [companies: company]
  end

end

defmodule MyApp.Company do
  use MyApp.Web, :model

  schema "companies" do
    has_many :company_chemicals, MyApp.CompanyChemical
    has_many :chemicals, through: [:company_chemicals, :chemicals]

    field :name, :string
  end
end

defmodule MyApp.CompanyChemical do
  use MyApp.Web, :model

  schema "company_chemicals" do
    belongs_to :chemical, MyApp.Chemical
    belongs_to :company, MyApp.Company
  end
end

使用这些模型,MyApp.Chemical.with_companies/1工作按预期返回一个查询,该查询将生成带有填充:companies字段的化学品,但我试图通过关联表以编程方式预加载字段来创建类似以下的函数:

def preload_association(query, local_assoc, assoc_table, assoc_table_assoc) do
       from  orig in query,
  left_join: association_table in assoc(orig, ^assoc_table),
       join: distal in assoc(association_table, ^assoc_table_assoc),
    preload: [{^local_assoc, distal}]
end

但是,由于该preload: [{^local_assoc, distal}]行,此函数将无法编译.

如何预加载一个has_many的关联？谢谢.

Answer 1

你是否以任何方式过滤你的联接？因为,如果你不是,你应该调用preload:

query = from c in MyApp.Company, where: ...
companies = Repo.all(query)
companies_with_chemicals = Repo.preload(companies, :chemicals)

要么:

query = from c in MyApp.Company, preload: :chemicals
companies_with_chemicals = Repo.all(query)

这将是快过因为它两个单独的查询,减少从处理的整体结果集的大小companies_size * chemicals_size来companies_size + chemicals_size.

请注意,您也应该能够加入has_many :through.