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Now we worked a lot with group and project and it's really important for you to understand the difference.

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Group is for grouping multiple documents into one document, whereas project is a one to one relation,

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you get one document and then you will return one document, that one document we'll just have changed.

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So for group, you have multiple documents and you return one grouped by one or more categories

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of your choice

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and then also with any new fields with some summary, well statistics or summary calculations, in projection,

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you have a one to one relation.

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So in grouping, you do things like summing, counting, averaging and so on, in projection phases, you transform

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a single document, you add new fields and so on.

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This is the difference and this is really important to understand and to get right.