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一个具有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 可以按预期工作，返回一个查询，该查询将生成一个带有填充 :companies 字段的化学品，但我试图创建如下函数，以通过关联表以编程方式预加载字段：

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

您是否以任何方式过滤您的联接？因为，如果你不是，你应该调用 preload 来代替：

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

或者：

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

它也会更快，因为它会执行两个单独的查询，将要处理的整体结果集大小从 companies_size * chemicals_size 减少到 companies_size + chemicals_size。

请注意，您还应该能够加入 has_many :through。