Ecto select 选择字段和返回值结构

我们在Ecto的文档中可以看到select的几种演示：

from(c in City, select: c) # returns the schema as a struct
from(c in City, select: {c.name, c.population})
from(c in City, select: [c.name, c.county])
from(c in City, select: {c.name, ^to_string(40 + 2), 43})
from(c in City, select: %{n: c.name, answer: 42})

遗憾的是文档里并没有给出它们返回的是什么样的数据类型，这里给出一些演示：

1. from(c in City, select: c)

返回 struct 列表，包含所有字段

query = from i in Info,
             select: i
infos = Repo.all(query)

[%Confer.Info{__meta__: #Ecto.Schema.Metadata<:loaded, "infos">,
  content: "华北电力大学", id: 1,
  inserted_at: #Ecto.DateTime<2016-08-22 07:57:15>, name: "举办地址",
  slug: "address", updated_at: #Ecto.DateTime<2016-08-22 07:57:15>},
 %Confer.Info{__meta__: #Ecto.Schema.Metadata<:loaded, "infos">,
  content: "010-88888888", id: 2,
  inserted_at: #Ecto.DateTime<2016-08-22 07:57:15>, name: "联系电话",
  slug: "phone", updated_at: #Ecto.DateTime<2016-08-22 07:57:15>},
 %Confer.Info{__meta__: #Ecto.Schema.Metadata<:loaded, "infos">,
  content: "admin@thermophysics.cn", id: 3,
  inserted_at: #Ecto.DateTime<2016-08-22 07:57:15>, name: "官方邮箱",
  slug: "email", updated_at: #Ecto.DateTime<2016-08-22 07:57:15>},
%Confer.Info{__meta__: #Ecto.Schema.Metadata<:loaded, "infos">,
  content: "CNETD2016", id: 7, inserted_at: #Ecto.DateTime<2016-08-22 07:57:15>,
  name: "菜单标题", slug: "header_logo",
  updated_at: #Ecto.DateTime<2016-08-22 07:57:15>}]

2. from(c in City, select: [:name, :population])

只取对应字段，其他字段为nil，返回结构体列表

query = from i in Info,
             select: [:slug, :content]

[%Confer.Info{__meta__: #Ecto.Schema.Metadata<:loaded, "infos">,
  content: "华北电力大学", id: nil, inserted_at: nil, name: nil,
  slug: "address", updated_at: nil},
 %Confer.Info{__meta__: #Ecto.Schema.Metadata<:loaded, "infos">,
  content: "010-88888888", id: nil, inserted_at: nil, name: nil, slug: "phone",
  updated_at: nil},
 %Confer.Info{__meta__: #Ecto.Schema.Metadata<:loaded, "infos">,
  content: "admin@thermophysics.cn", id: nil, inserted_at: nil, name: nil,
  slug: "email", updated_at: nil},
 %Confer.Info{__meta__: #Ecto.Schema.Metadata<:loaded, "infos">,
  content: "http://placehold.it/150x150", id: nil, inserted_at: nil, name: nil,
  slug: "wechat", updated_at: nil},
 %Confer.Info{__meta__: #Ecto.Schema.Metadata<:loaded, "infos">,
  content: "2016 中国工程热物理学会·工程热物理学术年会",
  id: nil, inserted_at: nil, name: nil, slug: "header_h1", updated_at: nil},
 %Confer.Info{__meta__: #Ecto.Schema.Metadata<:loaded, "infos">,
  content: "2016年10月21-23日 | 北京·华北电力大学", id: nil,
  inserted_at: nil, name: nil, slug: "header_h2", updated_at: nil},
 %Confer.Info{__meta__: #Ecto.Schema.Metadata<:loaded, "infos">,
  content: "CNETD2016", id: nil, inserted_at: nil, name: nil,
  slug: "header_logo", updated_at: nil}]

3. from(c in City, select: {c.name, c.population})

返回map列表，并只返回对应字段

query = from i in Info,
             select: {i.slug, i.content}
infos = Repo.all(query)

[{"address", "华北电力大学"}, {"phone", "010-88888888"},
 {"email", "admin@thermophysics.cn"}, {"wechat", "http://placehold.it/150x150"},
 {"header_h1", "2016 中国工程热物理学会·工程热物理学术年会"},
 {"header_h2", "2016年10月21-23日 | 北京·华北电力大学"},
 {"header_logo", "CNETD2016"}]

可以使用 Enum.into(%{}) 转化为

%{"address" => "华北电力大学", "email" => "admin@thermophysics.cn",
  "header_h1" => "2016 中国工程热物理学会·工程热物理学术年会",
  "header_h2" => "2016年10月21-23日 | 北京·华北电力大学",
  "header_logo" => "CNETD2016", "phone" => "010-88888888",
  "wechat" => "http://placehold.it/150x150"}

进而使用 infos["adress"] 进行访问

4. from(c in City, select: [c.name, c.county])

返回列表的列表

query = from i in Info,
             select: [i.slug, i.content]
infos = Repo.all(query)

[["address", "华北电力大学"], ["phone", "010-88888888"],
 ["email", "admin@thermophysics.cn"], ["wechat", "http://placehold.it/150x150"],
 ["header_h1", "2016 中国工程热物理学会·工程热物理学术年会"],
 ["header_h2", "2016年10月21-23日 | 北京·华北电力大学"],
 ["header_logo", "CNETD2016"]]

5. from(c in City, select: %{n: c.name, answer: 42})

返回自己构造的 map 列表，map中的字段为query自定义

query = from i in Info,
             select: %{tag: i.slug, content: "none"}

[%{content: "none", tag: "address"}, %{content: "none", tag: "phone"},
 %{content: "none", tag: "email"}, %{content: "none", tag: "wechat"},
 %{content: "none", tag: "header_h1"}, %{content: "none", tag: "header_h2"},
 %{content: "none", tag: "header_logo"}]

一般返回列表之后，都是采用 for info <- @infos do ... end 这种语法遍历，得到对应的 map 或 struct 以后则可以使用 info.address 或 info["address"] 来 access。