按primary和secondary角色字段合并分组，统计各角色用户数量

Alright, let's figure out how to count those user roles properly. You've got a list of users with primary and optional secondary roles, and you want to tally up how many times each role appears across both fields—here's how to do it in a couple of common scenarios:

1. Using Plain JavaScript #

If you're working with the user data as a local array in JS, you can use flatMap to collect all valid roles, then reduce to count occurrences:

const users = [
  { _id: 1, roles: { primary: 'Cook', secondary: null } },
  { _id: 2, roles: { primary: 'Waiter', secondary: 'Cashier' } },
  { _id: 3, roles: { primary: 'Cashier', secondary: 'Bar Tender' } }
];

// Step 1: Collect all non-null roles from primary and secondary fields
const allRoles = users.flatMap(user => {
  const rolesList = [user.roles.primary];
  // Only add secondary role if it's not null
  if (user.roles.secondary !== null) {
    rolesList.push(user.roles.secondary);
  }
  return rolesList;
});

// Step 2: Count occurrences of each role
const roleCounts = Object.entries(
  allRoles.reduce((counts, role) => {
    counts[role] = (counts[role] || 0) + 1;
    return counts;
  }, {})
).map(([role, count]) => ({ role, count }));

console.log(roleCounts);
// Output: [
//   { role: 'Cook', count: 1 },
//   { role: 'Waiter', count: 1 },
//   { role: 'Cashier', count: 2 },
//   { role: 'Bar Tender', count: 1 }
// ]

Breakdown: #

• flatMap converts each user into an array of their valid roles (skipping null secondary roles) and flattens the result into a single list of roles.
• reduce iterates over the role list to build an object where keys are roles and values are their counts.
• Finally, we convert that object into your desired array format using Object.entries() and map().

2. Using MongoDB Aggregation #

If your user data is stored in MongoDB, you can use an aggregation pipeline to compute the counts directly in the database:

db.users.aggregate([
  // Create an array of non-null roles for each user
  {
    $project: {
      roles: {
        $filter: {
          input: ['$roles.primary', '$roles.secondary'],
          cond: { $ne: ['$$this', null] }
        }
      }
    }
  },
  // Split each role in the array into its own document
  { $unwind: '$roles' },
  // Group by role and count occurrences
  {
    $group: {
      _id: '$roles',
      count: { $sum: 1 }
    }
  },
  // Rename fields to match your desired output format
  {
    $project: {
      _id: 0,
      role: '$_id',
      count: 1
    }
  }
])

Breakdown: #

• $project + $filter creates an array containing only non-null roles for each user.
• $unwind converts each role in the array into a separate document, making it easy to group.
• $group tallies up the number of times each role appears.
• The final $project renames _id to role and removes the original _id field to match your requested output structure.

内容的提问来源于stack exchange，提问作者Combustible Pizza