This guide demonstrates how to use field setting to map multiple columns to a single field.
For more detail about the manyToOne setting, see the reference page.
Suppose we’re collecting tax information: email, primary_earnings, and optional additional_earnings. Users upload spreadsheets with these columns, though the number of additional_earnings columns may vary. We’ll map them all to a single additional_earnings field and validate all earnings as USD currency.

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Field Configuration

Adding the property manyToOne with the value true enables many to one mapping. 
export const fields = [
  {
    label: 'Email',
    key: 'email',
    type: 'email'
  },
  {
    label: 'Primary Earnings',
    key: 'primary_earnings',
    type: ['number', 'usd'],
  },
  {
    label: 'Additional Earnings',
    key: 'additional_earnings',
    type: ['number', 'usd'],
    manyToOne: true,
  },
];
With this setting enabled, a user will be able to choose multiple columns and match them to the field without error. many to one column mapping

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Data Review Screen

With multiple columns mapped to one field, all the columns will have the label with a number indicating their order. Note that validations defined in a manyToOne field apply to all mapped columns. data review screen with many to one

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Results

Upon successful import, the results and metadata include all the values from the mapped columns and metadata indicating how the columns were mapped to the target schema.

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Result Data

Each value from a manyToOne column appear in an array that is the value for the manyToOne field’s key. 
[
  {
    "email": "tumvu@comowa.fk",
    "primary_earnings": 8954.08,
    "additional_earnings": [
      66.66,
      88.88
    ]
  },
  {
    "email": "hovawu@goiv.io",
    "primary_earnings": 446.02,
    "additional_earnings": [
      6478.45,
      7435.89
    ]
  },
  {
    "email": "idmup@te.au",
    "primary_earnings": 2175.84,
    "additional_earnings": [
      7949.13,
      1998.52
    ]
  }
  ...
]

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Result Metadata

The result metadata indicates which columns from the source file were mapped to which columns. 
{
  "id": null,
  "filename": "earnings.csv",
  "importIdentifier": "earnings",
  "user": {
    "id": "testUser"
  },
  "rowsWithError": [],
  "rawHeaders": [
    "email",
    "salary",
    "gifts",
    "bonuses"
  ],
  "fields": {
    "email": {
      "fileHeaderIndex": 0,
      "fileHeader": "email",
      "isCustom": false,
      "manyToOne": false
    },
    "primary_earnings": {
      "fileHeaderIndex": 1,
      "fileHeader": "salary",
      "isCustom": false,
      "manyToOne": false
    },
    "additional_earnings": {
      "fileHeader": null,
      "fileHeaders": [
        "gifts",
        "bonuses"
      ],
      "fileHeaderIndex": null,
      "fileHeaderIndexes": [
        2,
        3
      ],
      "isCustom": false,
      "manyToOne": true
    }
  },
  "errors": []
}

​
Hooks

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Row Hook

manyToOne fields can have their values changed and their info messages and validations set in row hooks. 
dromo.registerRowHook(function (record) {
  if (record.index !== 2) return record;
  record.row.additional_earnings.manyToOne![0].value = 42.0;
  record.row.additional_earnings.manyToOne![1].info = [
    { level: "error", message: "Invalid additional earning" },
  ];
  return record;
});

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Column Hooks

For manyToOne fields, a column hook callback registered for a manyToOne field will be invoked once for every mapped column. 
dromo.registerColumnHook("additional_earnings", (values) => {
  // Cap additional earnings at $5,000
  return values.map((v) => {
    if (v.value > 5000) {
      return {
        ...v,
        value: 5000,
        info: [{ level: "info", message: "capped at $5000" }],
      };
    } else return v;
  });
});

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Step Hooks

Step hooks REVIEW_STEP and REVIEW_STEP_POST_HOOKS can be used to inspect column mapping at run time to see which columns were mapped to oneToMany fields. The full data object supplied as an argument to the step hook will resemble this: 
{
  "fields": {
    "email": {
      "fileHeaderIndex": 0,
      "fileHeader": "email",
      "isCustom": false,
      "manyToOne": false
    },
    "primary_earnings": {
      "fileHeaderIndex": 1,
      "fileHeader": "salary",
      "isCustom": false,
      "manyToOne": false
    },
    "additional_earnings": {
      "fileHeader": null,
      "fileHeaders": [
        "gifts",
        "bonuses"
      ],
      "fileHeaderIndex": null,
      "fileHeaderIndexes": [
        2,
        3
      ],
      "isCustom": false,
      "manyToOne": true
    }
  },
  "rawHeaders": [
    "email",
    "salary",
    "gifts",
    "bonuses"
  ],
  "headerMapping": {
    "email": "email",
    "salary": "primary_earnings",
    "gifts": "additional_earnings",
    "bonuses": "additional_earnings"
  }
}
So in code: 
...
dromo.registerStepHook("REVIEW_STEP", (_uploader, data) => {
  console.log("manyToOne file headers", data.fields.additional_earnings.fileHeaders);
  // ["gifts", "bonuses"]
  console.log("manyToOne file indexes", data.fields.additional_earnings.fileHeaderIndexes);
  // [2, 3]
});
...